I second correcting the operating pressure. I would put it it at 250-300 psi to start off then adjust the IVG accordingly. The next thing I would check is if the pressure stays the same from shot to shot. A spread of 270-277 FPS is pretty good, so I don't think you should see a problem caused by regulator consistently. After that, I also agree on under boring or over boring your paint.
For feeding issues, in addition to checking the ball detents, check how far back the front of the bolt travels inside the breech at full pump stroke. If it's going too far back behind the feed neck opening, then you could get double feeds or jams. For misfeeds, if you are still getting them after you have corrected the IVG setting being too far in, make sure you are getting a full pump stroke without any sticking points.