It happens on most roads, I've lessened it a little by adjusting the spokes and reducing the flat spot, it was about 3-4 mm out of true , now it's down to around 1mm so when I get hold of a torque wrench I'll deal with it properly. I just wondered if it's old hat suspension or perhaps a damping issue - thicker oil perhaps ?. I haven't delved into the replacement of oil yet but will probably look at this at some point.