94 honda overheating

my wife's 94 honda dx with a manual transmission has been running hot, but not overheating until last week. i was running the air on high with the car idling on a hot day, and it actually spilled some coolant. to this point she was not losing any coolant, no drips etc.

i changed the thermostat and fan switch for the radiator fan. it was still running hot, but not overheating. i took the car to a local car repair service. they said first the coolant sensor switch was bad. supposedly they replaced it. now, they are telling me the gauge is bad. they said their diagnostic equipment states the coolant is not overheating, the engine is fine & the water pump but now the gauge goes all the way up past the red when the car is on for a few minutes.

when i got the car home, i noticed some coolant on the top of the radiator. washed it off. when my wife came home today, there was more coolant on the top of the radiator. i changed the radiator, new cap, new coolant etc. with the radiator change, the gauge is not getting as hot, but still spilling fluid on top. the fan does not seem to be cycling on and off consistently. i also replaced the fuses and checked for antifreeze in the exhaust or oil in the coolant. help!