HVAC Mixture Control Slide Problem

I have a '97 Civic HX. The HVAC mixture control slide doesn't work properly. It will slide to Hot or Cold but it will not stay in the middle
when the blower fan is on high.
If the blower fan is on low, it works correctly. As the blower fan speed is increased, the slide gets more difficult to place in the middle. The wind from the blower fan seems to be pushing on the adjustment door inside the underdash heater box and causing the problem.
With the blower fan on high, it is impossible to get the slide to stay in the middle. If the slide is at Hot and I start to move it toward Cold, the slide moves easily for 1 inch then gets difficult to move. If I let go of the slide then, the slide moves back toward Hot. If I continue to move the slide toward Cold (with increasing force) the slide will come to about 1/2 way (between Hot and Cold), then jump 1 inch or so toward Cold. A similar thing happens if I start at Cold and try to move the slide to the middle. I would describe this phenomenon as a 'toggle'. The slide will stay at Hot or stay at Cold but there is a large range in the middle where it will not stay.
I've disassembled the dash and removed the heater box. I expected to find something broken inside to cause this. I can't find anything broken. I wonder if all '97 ish Honda Civics suffer from this. I understand that '96-2000 civics are all the same. People with '96-'00 Civics; can you comment whether or not you have experienced this problem? Any insight would be appreciated.
Thanks, Andy.
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