Highly inconsistent playing level I have a very inconsistent playing level, which is very strange to me. Sometimes I'd make an almost-brilliant move (usually a tactical sacrifice, which once made an IM stare at me with awe) and other times I'd make a beginner blunder that would shock a 1400 amateur. I'm unrated although my trainer estimated my Elo to be about 1850. I've learnt chess two years ago and studied it seriously several months earlier. I've since been studying chess every free hour I've got. What is the cause of my unusual problem ? Is it inexperience ? A health issue ?
A standard piece of advice that used to be given to inexperienced players was to always write your move down before playing it and in the gap between writing it down and playing it to have one last quick check to make sure it was sound. The new (post July 2014) rules no longer allow this but the thinking behind the advice still applies.

Your computer is always consistent, unlike you. The reason is that it is inanimate silicon whereas you are a human being. You are driven by emotion. When you make a bad move you do that because your emotions temporarily take control. You need a routine to reduce the emotional disturbance to your thinking.

The first step is to try and never make an immediate response to your opponent's move. Try and always write his move down first. Try and have a checklist of questions you go through before making a move. Things like:

Why did he make that move?

What is he threatening?

Do my plans have to change to take account of what he just did?

There are probably more questions like this that you or your coach can come up with.

You must also try and not be affected by how strong (or weak) your opponent is or how strong you think he is. Once you have calculated what you think is the best move then play it. Don't change your mind and play a tricky but possibly bad move because your opponent isn't so strong and you hope to trick him. If you've calculated that the best move is a difficult, tricky move but your opponent is much stronger don't chicken out and play a weaker "safe" move.

I know from my own experience how difficult this can be especially when I am playing a much stronger player. The fear can be very strong but always try and play the position on the board not the other player or the other player's rating.

This way you will reduce your bad moves and become a better player.