Here's what I'd do: check (clean) the air filter. Clean the carb -- I like compressed air for getting all the passageways clear. And make sure there isn't some crud at the bottom of the gas tank -- pull the fuel line off and let a little gas flow out into a clear container -- it should be clean.
That's a start -- let us know if it works.
BTW, what's the model number? And post some pictures if you have a chance.
Surging is usually a sign of an engine running a little too lean. You might try checking all your fuel lines to make sure they're clear and also the fuel shut off. If they're all unrestricted it's time to look into the carb.
Like everyone else has said, it's time to clean the carb. You'll get to where you can do this with your eyes shut if you fool with Lawn Boys very much. Great mowers, but just one of those things you gotta do to 'em. Good luck!