I've heard a malfunctioning abs module can sometimes cause erratic rpm like this because it is a pass through for the vss data. I'd be curious to know if there are any abs codes, and if not, what the live speed sensor from each wheel looked like.
When I had a problem with my module, one wheel would bounce from 0 to 80 all over the place and my traction control and abs would go haywire!
Probably not the problem, but I'm just curious why the abs light is on.