2001 2DR Blazer, 2nd owner, knew the first owner an history of it. 220k mile,well maintained,body and interior excellent condition. Has a random issue that I can’t fix. All of the sudden the trans will down/upshift in commanded, if at a red light it will fight shifting gears, rpm’s will rapidly swing up an down. Turn blazer off and wait couple of minutes and it will 70% of the time drive normal for a few minutes but will eventually act up again. Sometimes it goes all day before it starts acting up. Here is what over the last 14 months I have replaced: Trans totally rebuilt, my trans guy said other than normal wear it really could have gone another 50k, new fuel pump, new speed sensor, just changed the original poppet fuel delivery to the new injectors at the head system, distributor cap and rotor, new plug wires yesterday an checked plugs. Just drove it about miles in city traffic and ran perfectly, stopped at a store for bout 20 minutes, cranked it up and rpm’s sho to 3600, fought it all the way back home. Also I have removed and cleaned every harness ground point I can fine. Anyone got anything on this. I’m at my wits end here!

 

First off, have you pulled any OBD codes?

This is a long shot but I once had the cruise control go bad on my 92. It would not turn off once when I was going down the highway! I had to kill the ignition but leave the key in so that the steering would not lock and coast to the side of the road. I then disconnected the cable to the cruise control.