Sounds like either a bad sparkplug, dirty carburetor/fuel lines or old gas. If fresh gas and a new sparkplug don't cure the problem, remove the air cleaner to expose the carburetor, start the lawnmower and spray Gum-out in it until the lawnmower starts to bog-down. Let the engine come back up to idle speed and repeat the process a few times. If this works, add one capful of liquid
carburetor cleaner to a full tank of gas. This will help to clean the fuel lines.
If all else fails take it to a repair shop, but don't expect to get it back for at least a couple of weeks.