The temperature gauge on my Pontiac Montana has been 'bouncing' all over. As soon as the gauge starts going up from where it usually runs, I don't have heat...just blows out cold air. It will go up to almost the red and then slowly come down. There's been a few times it hits the red and a warning light comes on and I somewhat lose power. It's never actually overheated to the point that I've seen smoke. Sometimes it just goes up and down a bit, but when it does that I don't have heat. Help!