I would lean towards the final stage resistor being the problem. You can get a new aftermarket replacement FSR from $40-70 online, and a Genuine BMW FSR for around $125. I've even used the cheap no-name $25 FSR units with success, but it's not something I recommend.

With a screwdriver, you can change the FSR yourself in less than an hour start to finish.

If the CCU is bad, which I doubt is the issue from what you're describing, then known good used units can be purchased for under $300 on ebay. Once again, you can swap it out yourself in a matter of minutes.

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Eric Giles
'04 M3
'07 X5 3.0si
Many others.