Some things to check...
Vented fuel tank cap not venting,
Bad spark plug,
Grass, debris up in the flywheel houseing restricting movement of the governor
Clogged air filter,
You might want to post more information...such as, the make and model of the mower AND the engine. Sounds like it's probably a Briggs engine?
You might try some Sea Foam, available where ever auto parts are sold. It's a great fuel system cleaner and fuel conditioner/stabilizer.
Bob........54646 Central Wisconsin's most fun guy! Handy member since 3/14/'94