89 GP SE overheat or not

[jbicb3]

I have an 89 Pontiac grand prix w/150k miles. The temp guage started showing it was overheating, so I replaced the thermostat. Then the radiator began leaking, so replaced it, and was told by repair shop the water pump was bad. Replaced it, and the thermostat again. Radiator loses about a cup of water per week, sitting. No water in oil, or oil in water. When I run the engine, after about 10 minutes, the temp guage shows high temp (200) then drops back down, and up again. I can open the radiator cap at that time, and put my finger in the water. It's not hot. When driving it, the temp soars, then drops at random. The Low Coolant light also come on. When I check the coolant level, it is about an inch below the filler cap.