ICS Sig 552 problem, please help!

[chriskemp]

HI,
I have a problem with my ICS Sig 552, I have looked at the gun for hours and cannot find any fault. The problem is when the gun is on Automatic fire, is doesnt always fire the BB's and the gun makes a popping sound, it does fire, but every 5 or so it will spit one out or two will come out, but it will fire fine on single shot everytime. It sounds like the magazine needs wound up but I have 3 Mags with that gun so can rule out that fault. I have stripped the gun down and rebuilt it from scratch, I cannot see any sign of wear or damage on any of the parts. Any Ideas?

Chris