So replaced the thermostat cuz the heat wasn't working. Truck drove fine until today about 2 weeks after the fix. It's starting to overheat again and intermittent loses heat. Also the coolant is going somewhere. Not sure if there is a leak that I just can't see. Cuz there isn't coolant on the ground. Anyone have any suggestions. Or ideas

 

Welcome from Florida. Check to see if there is water in the oil. The oil will be milky white if there is water. It could also be a bad head gasket losing coolant into a cylinder and burning it away with the exhaust.

 

Occam's Razor suggests that the most like solution is the most obvious one. Which means look at what was the most recent change.

So I suspect a leak due to installation of that new thermostat. Also, it's possible that all the air wasn't bled out of the system when refilling with coolant. Don't overlook bad radiator cap causing loss through the coolant reservoir either.

Investigate: don't guess !!!!