I'm pretty certain that it's the converter. My gas mileage has suffered 3-4mpg and the rpms fluctuate when in OD. I'm think the lockup clutches are burnt up.

If this is true, how long until it takes out the trans?

 

More than likely, it's a 5-45RFE which is a 5 speed automatic. Therefore you would be in 5th gear TCC applied. When you let off the throttle & the rpm drops further than usual, it could be getting a bad signal from one of a number of sensors, or the computer may need to be reflashed with updated software version. I believe it's more than likely (90%) chance it's electrical. You stated you had HP tuners. I'm not familiar with that, but it sounds like it could be dropping out of gear or to a much lower gear, then when you get back in the throttle whatever is causing the problem causes the computer to increase the pressure and there's the bang when it goes back into 5th and/or lock-up(TCC engagement). These have a pressure sensor on the back passenger side of the transmission that has been known to have problems. It can give symptoms similiar to the TPS. I know this is definitely not what your looking for, but without driving it with a scanner and possibly a pressure gauge it's like finding a needle in a haystack especially without any trouble codes to go by. Maybe someone will message back that has experience with this exact problem. Hope you find something out soon. I'll see if I can find anything more.