I've had very similar on Hahnemuhle Photo Rag, and it was to do with the paper not being perfectly flat before loading into the printer.
Try laying your sheet on a flat surface beforehand, and if the corners are curling up at all then carefully bend/manipulate them in the opposite direction until the paper lies nice and flat.
Also worth checking the platten gap in the driver - I think I have mine set to 'widest' for photorag, but I believe others have sucess with 'wide' as well.
Doing the above certainly resolved the issues for me, hope it does for you as well!
Regards,
Andrew.