2000 EB Climate Control Problem

The climate control has always worked fine, however last week with it set to 72 as always, after driving about 30 minutes it started blowing massive amounts of hot air through the dash vents. Then it goes into meltdown mode switching between the floor vents and back to the dash vents as if it's trying to attain my set temp. These symtoms have been repeated everyday since the problem started.

I'm thinking there must be an interior thermostat that's malfunctioning. Can someone please provide some insight before I have to resort to a dealer visit?

Thanks,
Joe