The interesting thing about that is this. The faster you go, the more air flow passes through the radiator cooling the engine. Most vehicles will heat up going slow or at idle and not moving. So, either we have a restriction in the radiator or engine, or there may be an issue with a head gasket.
When you replaced the thermostat, did the engine block appear clean? Was the inside of the radiator at all corroded? By any chance, did you flush the system before refilling it?
I'm sorry I have so many questions. I'm trying to determine what has been done exactly so I know what to tell you to do next. Hope you understand.
Let me know.
Joe
Saturday, March 28th, 2020 AT 6:27 PM