Struggling to improve past intermediate level I have been playing a lot of chess recently after not having played much in the last few years. My rating (sorry, I don't know which rating systems) on most chess websites and so on tends to sit comfortably in the 1400-1500 range, but I can't seem to break that. On various tactics trainers, I seem to able to reach a tactics rating of around 2000 or sometimes higher. Again, I don't know how useful these numbers really are, but if I take them at face value then it would seem that tactics are not my problem. I also think my grasp of strategy is not too bad, I am quite aware of outposts, open files, weak pawn structures and so on and they tend to drive my moves in my games, more so than tactics I would say.

I have a couple of questions to ask regarding improvement here. Firstly, I play a lot against computers, since I can just play a game on my phone without worrying about internet connection. Is this a bad idea if I am trying to improve? The computer I play against seems to be totally brain dead below a rating of about 1300, then suddenly above that rating seems to make almost no blunders. It makes plenty of strategic errors, (advancing way too many pawns early on and stuff like that) but it just seems to be able to plug every hole so quickly, even though it creates a lot for itself. My question is: is it a mistake to practice against computers, given their somewhat erratic way of playing?

I think my game is okay in most areas, and I feel like it could be one or two crucial weaknesses that are holding me back. My opening and end game skills I think are adequate for the level I am playing at, but my middle game seems to be remarkably uninteresting. I can notice where the most important squares are and tend to avoid any big blunders, but against computers the games tend to seize up in the middle...they are just really dry in terms of attacking opportunities because the computer has got it all covered, and I can't create strong threats as quickly as it can defuse them.

I think this may be the big gap in my game. I can solve tactical puzzles quite easily, but I cannot consistently create those sorts of positions that appear in the puzzles in my own games. So my second question is: is this a common pitfall in players around my level and, if so, how do players usually break through this?

Thanks for any input, and sorry for the life story. :)
Unfortunately playing against computers is a mistake amateur players often make. 

There is not much to learn from computers as they are just that, computers. You will lose the psychological aspect which dominates a significant part of human chess. Computer's "plans" are based on brute calculation and not a  "feeling" for the position, as human players do. You cannot calculate to the level of a computer, nobody can, but you can obtain a "feel" for the position, and this can only be learnt by playing with strong human players.

That is the key, to play against strong human players, be it on the internet or in real life, preferably in real life. I myself never played against computers, and instead concentrated on playing on internet chess websites. But the greatest aspect of my improvement was my experienced gained from playing strong human players, in real over-the-board games. This helped me to absorb their ideas, their plans and to become an overall better player. 

Keep solving tactical puzzles, your doing good, don't stop! Tactics is a crucial part of chess and can help in transforming advantages from one form to another and to decide the game in your favor. Tactical Awareness can also stop you from making blunders and seeing your opponents threats before he executes them...

 "My opening and end game skills I think are adequate for the level I am playing at"


The endgame is a crucially important part of chess which amateurs always disregard. I cannot stress enough the importance of it. The legend himself Capablanca stressed the importance of the endgame. Improvement in the endgame will guarantee improvement of your entire game. Do not just neglect study of it because your endgame skills are "adequate for the level" you are playing at (1400-1500). Move forward. You neglect study of the endgame, and you wonder why you are not improving, well...

For openings its not as serious at your level. You just have to know the basic principles (control the center, develop pieces, castle quickly). Restrict your opening choice as well to make study of it easier. For example, don't play all 1.e4 1.d4 1.Nf3. Don't respond to 1.e4 with all 1...c5 1...e6 1...c6. Play just one opening! This will make it easier to absorb the ideas of that opening and will leave you more time to improve other aspects of your game.

Jeremy Silman's Complete Endgame Course is the perfect book for the improvement of the endgame at your level. I recommend reading up and practicing until the 2000 level. Do not be intimidated by the rating, it's just a number. Some of the material is challenging but I'm sure with repeated practice you will absorb it easily. 

OK I know I'm a Jeremy Silman fanatic but How to Reassess Your Chess is truly a great book which helped me to improve. Not trying to sell anything here, but it really is fantastic material.

To answer your question about creating positions that arise from puzzles

"is this a common pitfall in players around my level"        


Well this just shows that you are at that level. Common pitfalls of players at your level is missing tactical combinations, implementing the wrong plan (bad judgement of the position) and ignorance of the endgame.

Relating to your previous question

"how do players usually break through this?"


Break through the common pitfalls of players at your level?

Perfect question to finalize my post, as I mentioned before.

1.Play against strong(er) players (preferably in real life)

2.Study the endgame

3.Continue practicing tactics!

4.Read How to Reassess Your Chess Or take a look at this

OK just going to recommend a bunch of websites to help you

Chess Tempo 

The Chess Website

Chess

Internet Chess Club

Chess Network

Chessgames