last week my dodge ran hot while driving slow... faster i went the temp gauge would go down.. turned on heater fan to keep some heat out and was able to get car home. no signs of overheating other than idiot gauge on dash.. removed thermostate and checked for problems, thermostate opened up in pot of water so was not defective... put back in and started engine again. thinking fan not working... nope that was running. was fine on coolant... took to mechanic, they checked it out. everything was fine.... replaced thermostate thinking it might be sticking.... now runs fine until you are about 85 on highway... and temp gauge will start to climb... any ideas? sending unit maybe? car has about 300,000 org. miles.. most org. parts... replaced water pump about 4 years ago..