My 2002 750 spirit was puchased used in 2006 with 7,000kms. It had new tires at the time of puchase. When riding on smooth road surfaces the bike developes a rythmic hop. That is, its a smooth ride for aprox 50 meters then the bike hops for a few meters, then its smooth again etc. etc. I've had the bike checked out at the shop that gave it it's safety certificate and they could find nothing wrong althought they agreed with my complaint and tried installing another set of new tires. What could be the problem???? The wheels have been balanced and according to the shop are within spec for roundness.