I had this problem with a new, never fired Gen 3. It came out of the box with the ejector dinged. I realigned it, and it's been flawless since.
With the barrel/slide off the frame, slowly insert a fully loaded mag and watch the clearance of the round and ejector. The mag should lock just as the round closes the gap at the ejector, and the ejector should be parallel with the round. The gap, with mag locked in should be no more than the thickness of a piece of paper. The round should also not contact firmly with the ejector. Ideally, when inserting a full mag, the round should just "touch" the ejector, then slack off slightly (paper thickness gap) as you release insertion pressure on the mag. If the ejector is in contact with the round and under pressure, it will cause feed issues. If the ejector is misaligned, it can cause feed and ejection issues. If the problem is primarily with more rounds in the mag, and seems to be less often with fewer rounds in the mag, the top round is probably under pressure against the ejector. If you look at your fired brass and they have a scratch along the side, from about the middle toward the rim, the ejector is dragging on the case, and probably the problem. Post a pic of the top-down and side views of the ejector with a full mag inserted and locked.
Also, check the fit of the rim of a round with the extractor. A case should slide into position under the extractor with little effort, and should firmly engage the extractor. The extractor should hold the round in place when the case is in battery position (centered in front of the firing pin). If the extractor is too tight, it may be pushing rounds out of position as the action closes, exaggerated by the higher pressure of a full mag. If this is happening, the breach is dirty or damaged, or the extractor/spring/pin may be fouled or damaged.