oh yes i forgot to say, there is no engine codes or lights.

Hello guys, I own an s60 2001 I got a weird problem with I can't figure how to fix or what is it related too When I drive steady between 60-80 mph, for few seconds, my rpm starts to jump/dance by 200-300 till I either increase gas or not push at all. It does not happen above 80 or under 60. I've cle...