Grease doesn't go bad. You don't have to clean your gearbox regularly to make it work. I've read and MET people who have had guns stock for YEARS without opening them up.
DON'T get systema internals...........in fact just stay away from anything systema (except hop up rubbers, those are ok for some). Ever heard of the systema 'revolution' gearbox? Yea real big revolution for something that doesn't even work
To the OP, most likely dirt or small shavings of your gears. If you want better advice take pics. Clean your gearbox up, and inspect EVERTHING for chips, cracks, etc... before you put it back together................oh and BE CAREFUL about the trigger......once that bad boy pops out your gonna have a lotta 'fun' getting it back in.
Gills wrote: lube everything that has a flat surface.
Sorry I can't let bad info spread. DO NOT LUBE EVERYTHING WITH A FLAT SURFACE. You'll be wasting lube, you won't have enough grease on high contact points, and the extra grease will DO MORE harm than good. Lube goes on:
-the teeth of your gears
-rails that your piston rides on
-VERY CONSERVITIVE amount on the rails where your tappet plate moves
THATS IT. Anything else is just over doing it.