Q: I have a 00 deville I just changed the water pump housing and water pump the thermostat and radiator the upper and lower radiator 2000 Cadillac De Ville

I have a 00 deville I just changed the water pump housing and water pump, thermostat 'radiator and the upper radiator hose. The car idels for over a hr with the heat on full blast without overheating at all. But soon as I drive it starts to read its overheating to its full capacity and to shut off engine. But the crazy thing is the coolent isn't bubbling out of the resevoir. AT ALL! My mechanic did the engine block test. It's not cracked anywhere nor is it the headgasket. We also flushed the coolent system out. All that has been checked... Cause that's the first thing ppl put out there mouth. ITS UR HEADGASKET... smh But that's not the case and me and my mechanic can't figure this out. I definitely need some assistance on this one here. I also checked all the lines but maybe Im missing something. These northstars are something else. I had this car for over 5yrs and has yet to have any major mechanical issues with it which has been a blessing dealing with a northstar. Thanks

My car has 200000 miles.
My car has an automatic transmission.