This problem started after 200,000 miles. The car also started overheating at this time and saying that I had low coolant but there were no leaks. My guess is it's the intake manifold.

not stable heat temperature

I replaced the radiator myself. All is fine there, no leaks. the top hose is hot so the thermostat seems to be fine. the temp gauge however, fluctuates. When the needle is about a quarter of the distance the heater blows hot air. When the needle is half way (the middle ) it blows cold air. this fluctuation occurs several times while I'm driving.