Temp gauge running hot when I slow down, is it the thermo, or maybe a rad flush

I Would change the thermostat and flush the system that is what I did on my brother in-laws 5.4 because it was running cold and would not smog...as far as a warmer thermostat I’m not sure it would hurt any as long as it does not over heat

it could be you have an air lock in the cooling system
remove the rad cap from the rad, start the engine and let it run until the coolant drops (add to fill it back up) or until the coolant boils out of the rad spout.

maybe pull the heat sensor. -replace it OR clean it off, reinstall it.

If running ok at idle and not running hot, then I'd first examine my rad hoses very carefully, especially lower rad hose. If old and soft, easy to squeeze then replace it/them. Also replace rad cap as well. A soft hose may be collapsing when engine RPM increases, and restricting flow through engine. Next would be to observe water flow with engine running to be sure water pump is doing it's job. I assume you have cleaned and checked rad already. Note: if you run at highway speeds and it gets hot, try turning heater on high fan and hot setting to see if temperature gauge goes down at all, which would make me lean toward a faulty radiator.

replace the thermostant again this time get a hotter temp thermostant and buy it from the dealer, the other thing is that these cars are known for the temp sensors being bad this would give you the false reading on the gauge but would not affect the heat, if its taaking a long time then i would replace the stant and this time or before replacing the stant i want you to flush out the heater core, remove the hoses from the heatercore and use a water hose to flush water through the heater core, a plugged core will not throw heat or will start to allow water to flow when it gets good and hot to flow through the retricted core due to rust and crud, try the flush first and then try a new temp sensor if the flush helps the heat, hope this is helpful good luck.

Make sure coolant level is good and no leaks. Check that belts are there and tight( when you push down on belts with finger it should only move about half to one inch). Also check temp. with hand held thermometer at ends of rad. hoses at the motor. Temp reading should concur with car gauge ( if not gauge maybe faulty) If all is good consider thermostat replacement ASAP

It sounds like the cooling fan is not working, with the motor off look in front of the motor and there is a fan or 2, check the wiring and plugs, some times they corrode and can be cleaned with alloy wheel cleaner then water then spray oil in them otherwise the fuses and the thermo switch need checking.

With what you describe the problem to be I would have the radiator flow checked. Sounds like it may be clogged if not the pump is probably bad. The head gasket should be o-kay due to no loss of fluid or smoke out the tailpipe. Do theses checks and you should find your problem.

check and clean evap and radiator, check level and ratio of fluid

Make sure you're using proper amount of coolant in the water. You could have dirty fins or something blocking the front of the rad preventing it from cooling enough or might be rad clogging up. If gauge goes over half way then I'd worry. I'd remove rad and use rad cleaner or get it cleaned professionally.

try changing the fan clutch if you dont have any leaks. make sure there is nothing blocking the radiator or condensor. good luck