I have a 2005 Jetta TDI 1.8 with the Triptonic (sp) trans, and for the last month, as it goes from one gear to the next, the RPM's shoot up, and the trans shudders into the next gear.
It will not do it every trip, only intermittently. I took it to the shop, and they scanned the car, only to come back with temp sensor bad, Engine High temp coolant circuit malfunction, and thermostat needs replacing.
Stated that the trans was poss getting too hot.
I do have some foam in the front grill (partial) to help the car heat up faster, as the diesel will not put much heat into the cabin if I don't.---I do NOT have the lower grill covered.