99 Silhouette with a coolant issue

The motor runs smooth. No white smoke from the exhaust, no coolant leaks on the motor anywhere. But the motor starts overheating if its ran more than 5 min or so, and theres no heat inside the car. We put a new thermostat in it today and bled the coolant lines and now it has no heat til it gets a little over halfway on the temp gauge, and then the gauge starts dropping down, and the heat works, until it bottoms out around the first line on the temp gauge. Then the gauge slowly starts climbing again and the heat fades away.

anyone have any ideas? Like I said theres no visible leaks on the motor, no coolant in the oil, and no white smoke from the exhaust. So I thought I had head gaskets eliminated, but mechanic says it must be head gaskets.