I found that taking the slack out of the cush drive on my bike cured surging completely. I just cut 4 strips of reinforced 5mm 'belt' rubber thenplaced them against the drive side rubbers. Then I lubed the rubbers with rubber grease. After a reasonably hard 1700 km, mostly fully loaded, there is still no slack and no surging. However, those overworked dampers are stuffed and should be replaced.