I have recently purchased a used wisper 705 torque for my wife its in immaculate condition only showing 640 miles on the display. However it seems to have a small fault when she uses it it zips along nicely but it then surges in power its not all the time but in sporadic bursts like its giving 80% then 100% power, can anyone suggest what could be wrong and how best to rectify. Shes very pleased with the bike its practical and comfortable and it fits her lifestyle perfectly just we would like to sort out the little niggle.