Heater blows out (at times) cool air instead of.

Heater blows out (at times) cool air instead of heat. It comes and goes. The blowers works at the right speeds; however, sometimes when it is set on "hot" it blows cool and then it will blow hot, and so on. I was told that it has something to doo with a flap behind the dash board that sticks causing this problem. Is there a way to fix it myself to avoid the $400 repair bill?