1. overheat: is it losing coolant, boiling over?
2. ^ an air pocket will cause this problem.
3. engine cold, open radiator cap and let engine run until thermostat opens, watch for bubbles in fill opening. Rev engine a few times to push out any bubbles. top off, THEN shut off engine, let cool.
weird problems:
1. water pump impeller blades are worn or it is spinning on the shaft
2. heater core is restricted
3. small leak from hose fitting allowing air to enter system.
4. leaking radiator cap or damaged fill neck.
5. new thermostat took a dump.
6. I have fixed this problem on some engine by drilling a 1/8" hole in the thermostat at the top edge.
7. too many bugs on your radiator/condenser
BTW: I am not guessing....I have seen, fixed all of the above.
weld on ..........[