2000 Pontiac montana idle problem?

[whtsthislif4]

it has a 3.4L engine. The problem is intermittent. Customer says it only happens sometimes but when at a stop the truck will rev up for no reason. She said she came to a stop in her driveway, Put the emergency brake on and the truck tried to pull forward while in drive. And when she shifted to park the engine rpms climbed to 3500rpm. I couldn't duplicate the problem. Yes it's fuel injected, And there are no codes coming up. Was wondering if anyone has run into this problem, or if there are any TSB's about this vehicle doing this..... Any help would be great... The only thing i saw on my scanner was that one of the O2 sensors wasn't fluctuating like it should. It was basically not doing anything and staying at one reading. And they usually go up and down the scale when working properly. Not sure where to start. tps? Iac?
I just don't want to randomly start replacing parts. With no code to aim me in the right direction. I'm thinking TPS . Hope someone can help, thank you!