I bought on ebay, bavauto.com, dealer and none seem to be a durable as the ones at 1aauto.com, I am not affiliated with them, just seems to be warrantied better and so far so good. Stock regulators typically last 5 years. You are exactly right, they drop without notice and I get enraged too, i was at a hotel and I told the valet NOT TO USE THE Windows, what did they do when they brought it back, the drivers window was down and they said I never told them. They bought me a dinner as it was on camera and I had a witness but still. What a pain, I live in AZ and I think it is worse here with the drying out of anything rubber or plastic.
Now I just replace the window regulators every 4 years. 1aauto seems to have pretty good prices and warrenty. I found them through ebay actually now that I think of it lol.

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Kevin
2001 X5 3.0i
Phoenix, AZ