sounds like its seizing up im afraid,who replaced the piston and did they check the oil pump as they do go wrong even if it is the electronic one which it is on this model.i would have the head off and have a look down the bore and up the exhaust prt to see the conditon of the piston,if a shop changed it then take it back to them,in the meantime add some oil to the petrol and shake it about a bit this will give it the lube its crying out for.
adam bike doctor, February 2010