I'd check the reg and relube it, just in case. I can't think of anything else that might cause a drop like that other than the reg. Of course, I'm not very versed on such things. It would seem to me that all the other parts associated with moving the air are in sync, so a bad timing situation would cause low velocity consistently. What about the fifth or sixth consecutive shot? Do they drop even lower? If you stop shooting for a moment and begin shooting again, is the first couple shots again at full velocity?