Sounds like the infamous final stage resistor. I just replaced mine this weekend. Climate control was "out" of control. Cost me about $60 delivered from Pacific BMW. Works like a champ now! Saved me about $300 from the stealer!
I wouldn't call the install simple, mainly because there is very little space to work with (passenger console side). You will need to remove the glove box and loosen another gizmo to gain access to the resistor. Oh, there is also an air duct in front of everything! Whole thing took me a couple hours because I managed to replace it by working around the air duct (instead of removing). I took some pics if you are interested.