Hi, I have a 08 colbalt ls and while I'm driving down the road at about 25-35 mph the tachometer wI'll k sometimes jump a bit from where it's at and will rise up about a quarter to half a rpm and go back down causing the car to feel like it's slightly jumping forward a few times. It will do it consecutively for a few seconds and then it stops doing it for a few minutes. This one's got me baffled, any help is appreciated.