Hello. I have a 2001 Bonneville. As i'm driving down the road, my temperature gauge goes down to like 150 or so. Causes me to lose heat. When my car just sitting idling the temperature goes up. I hit the defroster it goes back down while idling as well. The thermostat has been change twice. The engine coolant sensor been change. Think I got all the bubbles out. And its doing the same thing when i drive. Do anyone have any suggestions? Thank You.