After replacing the thermostat does the temperature gauge now read in the middle of the gauge once the engine warms up? I will send you an article I wrote for Repaipal. I would next look at the heater valve operation get a friend to operate the heater control lever to make sure the heater valve is fully opening. With the heater fully turned to hot both hoses at the heater valve should be hot or the heater valve or heater core is restricted. I have sent the article to the email address you have logged onto Repairpal with.