When you take it in to the shop, is it all warmed up and acting funny when you drop it off? When you do drop it off, are you just leaving the keys and waiting for a call? Sounds like the issue you've got has to do with trans temps. Next time, make sure the truck (transmission) is good and warm, and when you get there, leave it running and take one of the techs/service writers for a drive. If you leave it and wait for them to do it, it probably won't get warm enough for it to act up.