[L473ncy]

Are you using a hicap mag? I'm told that they can sometimes do that. Try using a midcap or lowcap mag (ie. one where you don't have to wind the bottom of the mag constantly).

Could also be the BB's you're using so try different BB's.

I would usually chalk to this kind of behaviour to mag problems so try different brand/capacity of mag to see what happens. If it stops then you've solved your problem but if it still happens you've got bigger problems on your hands.