1999 Oldsmobile Alero only overheats when started from cold

Engine Cooling problem
1999 Oldsmobile Alero 6 cyl Front Wheel Drive Automatic 90k miles

the car overheated twice today. What I have noticed is that it will only overheat if the engine is warmed up from a cold state. I started the car this morning after it sat overnight, after driving about 3 miles as fast as 50 mph it started overheating. I immediately pulled over, turned off the engine, let it sit for 30 seconds, then turned it on. After that it ran fine. Later in the day the same thing happened, the engine when first started was around 100 degrees F, started overheating, I pulled over, turned off then on the engine, it worked fine. Another symptom during these overheating events was that luke warm air came out of the heater even when the temperature knob was maxed out and the engine was over 220 F; not sure how a sticking thermostat would cause that.