it sounds like a bent crank to me.
you can easily check to be sure.
remove the sparkplug,ground the wire .
tilt the deck up so the spark plug end is up.
using a helper (ask the wife) get her to stand on the handle and press the brake bail and slowly pull the cord.
you will be looking at the end of the crank where the blade is bolted on,and visable wobble means its bent.
if it is it can be repaired but it might be weakened and any additional stress put on the crank could break it.
i have never seen one break but it is possible.
and the crank is not removed to straighten either.
many small engine shops have a straightener but wont advertise this.its a liability thing.
you might want to check out some in your area.
it will probably need a flywheel key as well.
all in all it will be an inexpensive mower.
any questions ...please ask
later chris
craftsman 10/28 snowblower with tracks husky 372xpg chainsaw sachs dolmar bc212 bushsaw mondo trimmer monster tractor with trailer cheep wheelbarro and couple shovels and a partridge in a pear tree