I have a 3.0L in my 89 Toyota 4 runner, I have heat to the vent, I installed a after market temp gage. (granted I installed it in the back coolant plenum). I flushed the coolant system from the temp sensor location to the upper radiator hose. I made sure there is no air bubbles in the system. I tested the wire to dash- when grounded to battery temp gage go's to hot. When idling vehicle run at 188-195 degrees on gage. When I start driving it slowly go's to cold and stays until I go back to idling for over 20 minutes like I just started it for the day. I also bought a temp gage hand held and check block when gage state it cold and block, upper radiator hose and heads are all at 198 degrees. I replaced thermostat as well. What could be causing my temp gage from telling me the engine temp? I know these engine our notorious for head gaskets issue. and I don't want to replace then if I don't have to. please help