2001 Ford Windstar Blows hot then cold

Greetings,

My 2001 Windstar just started this thing where as long as you have your foot on the gas driving the heater works great, come to a red light, and bam instantly blows cold air. Start driving again, gets hot again. Put it in park, goes cold instantly. I know with the old muscle cars I build you'd just change the thermostat and it's fixed, does the same hold true with these newer vehicles? Or is there something else wrong? Thanks for your help.