Okay so lets get this straight here.
-You opened the gearbox to change the spring
-You put it back together you had an FPS drop and a lot of misfeeding (wouldn't an FPS drop be expected after you change the spring...)
-You took it apart and put it back together and now it stops cycling after a few shots
When you say "stops working" do you mean it doesn't feed or it doesn't cycle?
How many is "a couple shots"?
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN