Update: figured out popping under load in all gears/misfire
Turns out the drive chain in the 4t60-e gets stretched out (in a blower car)over the course of 153,ooo hard miles and starts jumping on the sprockets. feels just like a misfire, sounds like popping thru the throttle UNTIL you stand outside the car when it does it. Gradually gets worse too, and won't do it with car sitting still. I guess I have to buy a SRTA now, unless anyone has a better idea on a less expensive rebuilt trans. GM will end up 1700.00 after tax. I guess other parts of the transmissions go before this happens in most cases.