1996 F-150 Inline Six. I bought this last August, great truck love it. The guy before me must of thought Leak stop was some new radiator coolant as there was a ton of it in there when I got it. After flushing, replacing the water pump ( the original leak the dude was to broke or lazy to fix) and thermostat, coolant it ran fine for three or four months then started to over heat on a 1 1/2 hour trip. I opened the cap, spilling out more of the leak stop that must have been lurking way below. Replaced the depleted coolant and it was fine for another month. Periodically the gauge would go past the middle of O and R. I'd turn the heater on and it would cool down. But lately not always. So Today I had it flushed by a mechanic (probably could have done the same if job) I drove 5 minutes to the house and it acted like it should. Temp goes to about R-M then drops to O-R. 30 minutes later after a short drive the gauge doesn't go down unless I turn the heat on. Its about 30 degrees out and the gauge doesn't drop like it should. Remember I changed the pump, thermostat, fan clutch in August. I checked the top hose (new) and it feels like its hot enough.
1. Do you think its the Thermostat again due to the Leak stop may not be totally flushed out even with a flush by the mechanic...may not have flush it loose from the thermostat? Much obliged for your replies....this is the only thing thats happening with this truck but its a "you know what"

 

When the truck is warm check the radiator at the top and bottom, if the bottom is cooler then there is still a bunch of junk in the botton causing a flow problem. This happened to my dad's Exploder and he had to replace the radiator.
Good luck!

 

to ya, haven't heard this way of checking yet. Sorry for the delay, its been the same issue since the first post, the turning on the heater method works for now so I just saw this jgger and will proceed. Three of three friends say its the radiator. I'll have to get the full engine flush before I replace the radiator otherwise I'll probably end up in the same sluge crap situtation again