I have a very similar sounding problem on my 90 300TE. The temperature and fan work fine for a while, but the side vents will go to full heat without any action on the controls. Fortunately the fan does not kick into high speed. Only means we have seen to correct it is to shut the climate control off for a bit.

It seems to be worse if the selected temperature inside is close to the outdoor temperature. Then again it happened several times last weekend when the spread was 30-40 degrees (inside 65-70, outside 30ish).

When it is hot here in TX, the AC works as expected.

Any additional ideas beyond vacum system etc. already offered?

Thanks - MRK