1999 Oldsmobile Silhouette Heat gauge goes up but engine do

The heat gauge goes up and and down, but the engine doesn't heat up. We have changed the thermostat. A mechanic showed me how to check and see if there is a bubble in the heat hose. When the radiator cap is taken off an you squeeze the radiator hose it overflows in the reservior, when it first started this did the trick for awhile but no longer works, also the heater only works part time, the service engine light goes on and then will go off and the low coolant light comes on and then goes off. Could it be the sensor or the water pump or what is going on?