I had the same problem on my passaat. I finally figured out that the piece that slides in the track was cracked in half and was causing the problem. Once it got real bad i just left it closed and eventually sold the damn thing. I am not sure if the BMW roof is made the same way (because i haven't had a prob. with mine YET) but it probably is. Look in the track for cracked parts or just try lubing it with lithium or spray silicone.
92' 626 ES
88' Mazda Truck
92' VW passaat (sold)
93' BMW 325is
86' BMW 318is
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