Old Passat B5.5 climatronic oscilates between hot and cold air at the vents. It usually works for a little while, then alternates between blowing hot and cold. You can hear the blend motor whirring away in the footwell as the temperature changes.

I did the diagnostic with no codes showing, and it also shows the correct outside and inside temperature. Cleaned the interior temp sensor on the unit, but no cigar.

Any more things to check?