tranny locking in and out

I am having trouble with my 99 gmc jimmy slt. I will be driving down the road at a constant speed and the tranny will lock in and out, which will vary the rpms. I brought it to a local tranny shop and they first over sized the valve body and replaced the torque convertor, which didn't fix it, then they replaced all the electical in the tranny itself and that didn't fix it, so they tried a whole new tranny and that didn't fix it. So now they are stumped and don't know what to do next. Any advise?