I fired up my Sears lawn tractor this morning and noticed after 10 minutes of mowing that the engine started missing and running very rough. It happens less with the blades disengaged, and more when under load. Only thing I did differently this morning than normal was check the oil and add a bit to bring the level back into the good range (was just a hair low). I've cleaned the air filter, but I can't determine what the problem might be. Any suggestions?