sounds like the blend doors aren't opening again like they should. Try what Fred said, and when you get a chance go on ebay and buy the correct year factory service manuals. there are troubleshooting flow charts to follow for a number of things, so pretty sure the HVAC system would have one. They are available on Ebay usually. You can get a set for well under $100 esp, if patient.
Then blend doors are vacuum actuated so you might be loosing vacuum and then they don't work right but I'm no expert on them