I have the same problem occasionally, it's the bonnet switch that's the cause, if the rubber cover is not on correctly, (usually if I have been under the bonnet for maintenance and touched the switch and not noticed) repositioning the cover to it's correct location cures the problem.

I had a simelar problem with my 300 TDi, it turned out to be the belt tensioner, (the large wheel at the top of the belt pullys) as this is a non servicable part, the whole unit has to be replaced. The job is quite simple, but before buying the replacement part, loosen the belt by placing a...