Hotend doesn't maintain temperature I got an Anycubic Predator last month, and after resolving a few mechanical problems, I was able to get it printing decently well. The only significant modification I've made so far is a set of 8-diode TL Smoothers, and I'm now mostly operating it via Octoprint.

However, during the last few prints, I've noticed the temperature dropping midway through the print. It warms up and cools down fine, but for some reason it's not able to sustain the temperature throughout the print.


In this case, the print started out at the correct temperature (200&nbsp;°C), held that temp for around 2 hours, then it dropped to a lower temp (174&nbsp;°C). It eventually went back up to the target temp, then dropped again 5 minutes later. I tried manually adjusting it to see if that could fix it, but no luck.

After this print completed, I restarted it to show how it is easily able to reach the target temp and hold it at the start of the print:



Any tips on diagnosing and resolving this issue?
Safety First
Let's look at the graphs. First: you should swap firmware for one that has Thermal Runaway, as, as it is, running about 15 minutes with 28 K less than the printer is ordered to work at is a clear indication that there is no Thermal runaway protection in place - it should have tripped over that long ago! But there is more!
Problem
But this graph and the lack of Thermal Runaway Protection also are typical for printers that have a design flaw: If the airflow from the part cooling fans or the coldend-cooling fan (that's the fan that always runs) brushes over the heater block, it cools it. This limits the achievable temperature.
Luckily, such is easily remedied in one of several ways:

Changing the airducts for ones that does not hit the heater block
Adding a silicone sock around the heater block
Kapton-tape and ceramic wool can be used to make a heater-sock too
Adding an air-shield in the shape of a bit of tinfoil can redirect the airflow away from the heater block, but make positively sure it is mounted Fire-Safe and can't be lost into the print!