my chevy has a tendancy to over heat only when idiling it has got up past 3/4 of the way to the redline and it looks like it is going to reach it but when i start to drive it stops rising and it drops to normal temp is this the thermostat or what could be the problem?

 

check your fan...

 

What kid of oil do you use?
What is the weather like where you are?
When was the last time your radiator was flushed ?
Check the fan and the water pump.