If you're lawnmower is running unevenly, check to see if they are any loose wires. Also replace the oil and spark plugs. Make certain the under carriage is clear of all debris. Try adding gas stabilizer or additive to the gas tank.
Make sure the gas you are putting in it has no water in it you may even want to add a cap of dry gas but no more than that because it ups the octane level in the gas and make the motor run hotter. And check the spark plug.