maybe we can trust the TM to be a bit more reliable?

Prepare thyself for the infinite rabbit hole of "fucking about with airsoft guns"!   Rubber parts, yes hop rubber and O-rings. If your AEG doesn't have an O-ring nozzle it may be worth getting a new nozzle that does (make sure you measure it first and get the right length). Again, all parts that @ak2m4 can help you out with. If the old grease is particularly heavy handed it may be worth getting some new shims and reshimming the gears once you clean it out. Check the bore of the cylinder for any scratching/gouges, make sure the piston head is still secure and that any pad on the inside of the cylinder head isn't loose/worn/damaged.   GBB is likely to be O-rings on the bottom of the mag too. Put some gas in it and submerge it to see if and where it leaks. No point changing fill valves if they're OK. Depending on the pistol you may be able to get a specific service kit for the mags (more expensive than just getting a bunch of O-rings but more likely to have the right ones).   Again, take pictures and post them up in the tech threads here if you're unsure!

That's a good way to get yourself kicked off site.