I had the same issue and traced it back to the clips I was using. They were cheap aftermarket clips that had significantly egregious Parkerizing on them. I polished them up, inside and out, with gray 3M pads and lightly oiled them. Now the gun feeds lickity split. Of course no such issue ever occurred when using honest-to-god GI clips.

"You can lead a man to logic, but you cannot make him think." Joe Harz
"Always certain, often right." Keith McCafferty