Aeg burst firing

[The Disavowed]

I really feel a little embarrassed to be asking this, but I haven’t touched an aeg for over 15 years and was never that good at taking them apart in the first place.
Dug out an old Cyma mp5 and thought I should make sure it is working properly and as soon as I connected the battery (still on safe) I got a burst of about 5 shots. When I put the selector onto single fire and pull the trigger it will fire a burst of about 5 shots. It does this every time the trigger is pulled. When I put the selector on continuous fire and pull the trigger it will fire a burst of about 5 shots. It will fire continuously when the trigger is held, but when the trigger is released it will continue to fire approximately 5 more shots. Each time the trigger is pulled it will fire a burst instead of a single shot.

I don’t really want to take the gearbox apart if I can help it, and I don’t really recall what part I should be looking at trying to fix. I am open to any suggestions anyone may have that I can try, preferably starting with something easy. I don’t even remember how to take the thing apart, I definitely won’t be swapping back from gas any time soon, but would like to get it running properly again if I can. I am pretty sure that this one was never taken apart before and I left it all standard as just a backup gun, and it has seen some use as it was always one I would let other people use, but as I say it has sat in a box for about the last 15 years, maybe more.  Nothing feels sticky or not working in any way that it shouldn’t, no extra resistance or stickiness on the trigger or selector at all.