Yep I've had the exact same problem with mine. The piston is seizing up in the bore, when it cools down it generally free's it's self up! Turned out to be oil starvation - try turning the oil pump up. Ignore the adjustment marks on the pump because when i lined them up the dam thing seized up when ever it go hot !(i.e. at high revs for any length of time.)
Alan
December 2008