I have a 98 Chevy Venture with a 3.4 engine and 89,000 miles. The engine goes from hot to cold(no rhyme or reason not just when stopping or idling) and occassionally the low coolant light comes on. The reservouir is full so I am not sure why it is saying that it is low on coolant. I checked the radiator and it is also full. A friend said that it may be air in the system that needs to be bled. Any help would be appreciated.
Simple things first. As you say the coolant is not leaking but the low coolant light is coming on, this would indicate that the switch in the header tank that indicates a coolant loss is faulty, so I would be checking that first. As for the over heating, replace the thermostat, and have the radiator pressure cleaned and see how things go from there.