As I headed to work this morning I waited for the temp gauge to climb up then turned on the heater. About the same time I noticed it wasn't hot air coming out I saw the gauge start climbing...fast. Right before it pegged the red, the heat started to come out and the gauge went back to normal operating temp? This happened on/off to and from work, but nothing overheated. Any ideas??? Oh yeah, it's a 98' 4.2L STX