I have an '89 Honda Accord, Manual transmission, around 190K. Recently replaced the Radiator, thermostat, upper radiator hose. Since then (although I may not have noticed it before) I have had a problem with the engine temps spiking up to the top, and the indoor heater only blows cold. Then the heat comes on inside and the engine temp drops. I have checked the radiator hose while the engine temp gauge is up, and it seems to be heating normally, so it doesn't seem like the thermostat, but I could be wrong. The fluid level seems fine.
You may have air pockets in the cooling system, to get this out just park on a steep hill, or jack the front end waaay up, make sure the engine is not already hot, and start the car with the radiator cap off. If this does not work make sure of a couple things: The radiator fan is working properly, the heater core hoses are not clogged (pretty easy to check, take both sides of the hoses off and put your garden hose up to one end) trust me, you will know if it is clogged. Last thing to check: there is a small coolant line that runs behind the intake manifold to a check valve. These rust out all the time. The hose then goes to where the intake manifold meets the head. Hope this helps.