sometimes my fan blower does not turn on and them after driving on highway speeds with the knob in the on position it will just spuratically turn on. I noticed today for the first time after experiencing this problem for about a month that the temp gauge started moving up high and back down to normal. once when this happened the heat went from warm to cold and then back to warm. What could be my problem? is my thermostat going bad? what could be causing my fan blower to sometimes work and sometimes not work?