Blowing hot and cold air alternately

My 2004 Highlander with 40,000 miles recently developed a similar problem to the one noted above (which I note has gone unanswered for a year). It doesn't matter whether I select A/C or fresh air ventilation, nor where I set the temperature selector, the air coming out of the vents varies from cold to hot sort of randomly as I drive. The service manager at our large local Toyota dealer said he had never heard of such a thing. He offered to have one of his mechanics take my car for a test drive, but I think the problem is not yet so frequent and severe that it would be noticed very quickly. It would be nice to know what parts should be tested to diagnose this problem!