cvdhwoe

Claus,

I just now saw your message.
This is from distant memory, so bear with me.
I disassembled our linear drive (Autohelm 6000) because it had a rough spot right in the middle. It wasn't particularly hard to do, but I'd definitely recommend you have lots of light and a good spot where you won't lose any of the balls when you take apart the recirculating part.
There is a part which recirculates the balls via a tube guide. We couldn't find anything wrong with the drive, and when we put it back together we used ep lithium grease. I suspect all it was, was dirt. It worked great after reassembly.

Sorry, but I don't have any diagrams or spares.