Originally Posted by roundel335
It's the regulator that's part of the fan unit - it sits up under the dash behind the glovebox area. You can get a new one for about $160 from getbmwparts.com and it includes the regulator plus the fan assembly itself.
Definitely could be the blower resistor. But before it's condemned, make sure there aren't any voltage issues when this is happening. Especially since you say it happens when at idle speed, it could be the consumer cut-off function reducing the blower speed in response to system voltage. When this happens, the blower will only go up to about half speed. I haven't seen these cars having too many resistor issues. Was a huge problem with the E46 and other models that shared it's climate control system.