When driving my 1999 GMC Denali for an extended period of time, say 30 miles or more, it begins to shift hard when accelerating. It will eventually turn on the SES light.
When I turn the truck off, even for just a minute or two, it will shift fine until I take another extended trip @ cruising speed (55-85 mph).
I have already had the transmission rebuilt. It didn't fix it.
Anyone know what is going on?