'94 22re running hot

I have an unusual problem where my engine temp. begins to rise whenever I have the heater running on low. I've never let it overheat, and turning the heater up full blast cools it off. No sign of leaking coolant anywhere, although I've had to refill my reserve tank twice in the last 5 months. The motor has 210,000 mi. on it. Has anyone encountered a similar problem?

Running the heater in August? Hope you're not in the midwest :-)

When does it do it? In traffic? on the highway? Possible problems (starting with least expensive) include thermostat, radiator cap, fan clutch, water pump or clogged radiator.

I'd start with a good flush of the cooling system and a new thermostat and radiator cap.

If the new cap and/or t-stat don't help, I'd have the engine compression tested and/or the cooling system pressure tested. You could have a blown head gasket. Yes, you can have a blown HG and no sign of a leak. When mine went on an old Corona w/a 22R I had the same symptoms. The leak was small and the coolant was being burned, wasn't even noticeable in the exhaust.

Also, what does the oil look like? clearish or milky?