Volvo 850 issues

#1

My 1995 Volvo turbo wagon, 153k miles, has a problem stumping 3 mechanics:

It intermittently revs the motor for no reason. It can happen while driving, but more often after a short drive to town after slowing to park. The motor revs to a high whine, I can usually slow it by either giving it more gas or changing gears. That was one issue, but it also nearly stalled the other day as well putting it in reverse after a 2 mile trip.

They seem incompatible, but there they are…I can’t drive it safely or with any confidence.

Just had a new exhaust and cat. converter installed, and the only work done for this new problem was a Volvo mechanic cleaned the throttle body, lube pedal and roller assembly. Any ideas? we’re stumped and ready to quit!!! Of course the problem never appears while a mechanic has it…just the owner, sigh.