Intermittent fan on A/C-defroster 318i

My son has a 96 318i that he has only owned 3 months or so. I had to drive this this evening and found that the defroster would not stay running during a bad storm and the car fogged up really bad. Very dangerous. My son tells me it has been this way since he bought it, but he had only run the A/C and when it acted up he simply used the sunroof and windows. I guess he planned on getting it looked at but no big hurry. Car was bought off an individual and was well taken care of and documented, but of course, there is no warranty here. It acts like the fan has either a loose ground, or perhaps a sensor is failing. The controls all function, but the fan will run say 10-20 seconds out of each minute, and speed varies from fast to slow when defroster engaged. Is this a common problem on these cars and how hard is it to fix (or expensive)? Any help appreciated. I do not have a service manual on this car as of yet...

This (common) problem is usually attributed to a capacitor in the climate control unit that goes bad:

http://www.macadamizer.com/bmwfix.html

Thanks for the information. Very helpful and I now see others having similar problems on here. I am looking at a couple of options to get it fixed. I can solder but I think we will take it out and send it out to one of the folks advertising repairs on Ebay. Do not want to chance putting wrong item in or ruining a board. However will still be a pretty cheap fix. These forums are very helpful. Thanks again!