Assuming the mower is a belt drive model (earlier models were chain drive), then the drive belt has to be slipping. Either the drive clutch needs adjusting (cable adjuster - some require cable to be pulled to engagen some require it to be pulled to DIS-engage - depends on when produced). Adjust it to make the drive more positive - whichever direction is appropriate. If the belt is tight, but still slips, it's worn - replace.
phil_saunders(a)bigfoot.com
Phil Saunders, September 2012