Fan speed always running at max.

I have an 1989 260E. A month ago, my heater fan developed a mind of its own. I'd be driving along quite happily when it would suddenly run at
maximum speed for a second or two, and then go back to normal. Initially, it happened a couple of times a week, then it happened more often, and now it would run at max. speed all the time (after the initial warm-up period).
It doesn't matter which of the max/min/auto buttons I push - it will always run at the maximum speed. It also happens regardless of whether I have the mode set at defrost, norm, or economy. The only way to slow the fan is to turn it off by pressing the off ("0") button, when the fan turns totally off. It almost seems like the system thinks it's on "defrost" all the time (when the fan normally turns to max). Does anyone know what the problem is, and if there are any work-arounds to fix the problem? I don't really care if defrost automatically turns the fan speed to high - I can always do that manually if required.
Thanks in advance for helping.
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