I bought a 2000 GMC Sierra 2500 about a year ago, it now has only 49,500 miles on it. It has the 6.0L V8 and 4L80 transmission. When I'm heading down the road at 55 or 60, it drives great and runs great, but it seems like it just doesn't want to stay in overdrive. It'll sit in OD until about 65, then it'll go into 4th gear and won't come back into OD unless I either come to a complete stop and go again, or shut the truck off, start it back up and go again...then it's the same story over and over again. I've had the transmission flushed and filled twice, thinking that maybe one of the valves might be clogged, but that isn't the case I guess. I'm going to try some Lucas transmission treatment and see what happens, but the truck has never towed anything in its life...Any suggestions?
