might wanna try reshimming it, motor could be dying or just need a cleaning, your battery could be the cause of it (eventualy they do die).
Depending on how many rounds you have on the gun the bushings coulda gotten worn out since their nylon and messed the shimming up regardless.
Also some greases (depending on who worked on it if it was ever opened) get thicker with colder weather (depends on your location of course) and could be contributing to it.
Could also be your motor height being in too far causing it to jam up a bit.
are your batteries NiMH or NiCad ?
I honestly think youd be find with stock TM gears, quite a few guys on my team had upgraded AEGs to 380-400fps on .25g and lasted quite a few seasons with no issues. We typicaly per person run through 10-20k rounds per airsoft season easily. But your problem still sounds like there is something wrong other than a battery issue, probably a good idea to tear it down and give everything a good cleaning and inspection.
EDIT: ^^^ I agree aznriptide , and if you stick with all the stock TM parts you shouldnt even need to adjust the AOE stuff...metal bushings is always a good idea though.