I believe you don't have a microfilter and that's the problem. There's an accumulation of junk/leaves/rodent corpses/twigs/etc. lodged in your blower wheels and in front of the evaporator. The only way to deal with it is to open up the blower housing and shovel the stuff out, vacuum what you can, and try to clean off the evaporator to the best of your ability. This situation was CHRONIC on my '89 535 and was one of the things that finally swayed me to an E39 with its huge and trivial-to-change cabin air filters.

One technique I found useful to clean the evaporator was a portable steam cleaner. It's hot enough to sterilize the moldy gunk and doesn't produce enough water to overwhelm the drain. The steam temperature is well below the decomposition point of R12 by the way, so nothing to worry about there.