Surging is often a fuel/air delivery problem. As others have suggested, when it stops running again, do a spark check to confirm it's not a coil or spark issue first.
I would suggest replacing the carburetor (about $80 list price) unless you feel confident enough to properly disassemble and completely clean the old one. If you'll get me the frame serial number off the tractor, I can get you the exact part number for the carb (and gaskets). Swapping out the carburetor is straightforward: