Idle goes up when car shifted into reverse?

[Sleeper]

I have a new gremlin. I'm not sure if this is tranny related, but it only happens when I put the car in reverse... The idle shoots up a bit? Enough to almost bog the motor and stall it out. It's killer on the "U" joints if I shift from "R" to "D". I had completely stripped the drive train, gutted and repainted the car, but the only mechanical change was when putting it all back together was putting in a different tranny (True BRF)
What else could cause this?
BTW. I have a poly mount on the DS and reg mount on PS

Paul