Check your final stage resistor which is located on the upper right hand side of the centre hump in the interior (for right hand drive - left of pedals). There are plenty of write ups about it. I recently had troubles with my E39 (same setup) and I bought a new final stage resistor only to discover that the plug/socket connectors were dirty and unplugging/plugging in a few times fixed the problem. If yours is intermittent it may only be dirty contacts.

__________________
2004 E53 diesel.
Titan silver on black
Sports package