I have a 2003 Chevy impala LS 3800 motor ... anyways, the RPM's tack will bounce while in overdrive and seems to be random at every time; It could be weeks on end but then it could do so everyday. It only happens while in overdrive and tends to stop when shifting into 3rd gear. I have replaced pretty much the entire car; oil pressure sensor, radiator, cooling fan, MAFS (mass air flow sensor), steering pump, plastic elbows alternator, and fuel filter (if that helps). The car was $700 and in bad shape not really road safe until i got parts. So i'm not really to sure what all has been done to the motor itself since before I owned it.
Another note: why does my car start, struggle to stay started then die after being ran for a bit? Like if i go to Walmart and shut off the car, I have to wait half hour to 45 minutes later for the engine to cool. The dash and fluids are fine but its like it struggles to stay "alive" after being ran. I've gotten into a habbit of just leaving my car on if i have to run into a gas station or something in fear of it not restarting ... could that be a crank related issue? I'm not really car smart but i can tell you if something isn't right.