I'm having the exact same problem you are. It is in fact my heater VSV. Start the car, turn on the heat and push up on the little little valve bracket thingy. When depressed mine will stay up and allow me to have heat until I turn down the temp below 85 then it'll close and I'll get cool air again.
I have a WTB thread in the respective section, but nothing so far! I'm thinking I'll just pony up the $150 and buy a new Toyota one.