As it happens, I picked my 300tdi up today and have exactly the same problem!! So, you guys think u.j? I'd guess against wheel balance as it does it nearly every time BUT not every time (like wheel imbalance would). Hope one or other of us works it out without spending too much!
Also; What's the alarm that goes off when I hit 75 mph!? it's pretty quiet but I haven't a clue what it's all about. Anyone know? ('98 disco "avimore").