Walk Behind Lawn Mowers

After just about 2 months of use, the engine started running roughly.  It starts well and runs smoothly initially, but after perhaps 10-15 minutes of use it sounds like its repeatedly about to stall.  Then if I stop and restart it, it's hard to start, taking many pulls.    When it finally does start after repeated tries, I get a puff of white smoke and then it runs, but again is rough.   The air filter was dirty so I replaced it.   I checked the oil level while cold, and the main line of oil is at the full line, but a line of oil runs up the side of the dipstick about a half inch higher.   Help!