look at the blade for damage. If looks ok check balance. Note its easy to bend the motor shaft if blade hits a hidden stump or something. Spin blade to see if shaft is bent.
Brian
April 2010
Sounds like the blade needs sharpening and balancing....take to your nearest lawnmower shop, will not be expensive.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.