I have a Scag zero turn walk behind commercial mower around 10 years old, 16 HP Vanguard engine. It starts fine, idles fine, even runs pretty good when the blade is engaged most of the time. After a few minutes of the blade being engaged the motor sputters terribly. I can disengage the blade and it smooths out. I've changed air and gas filters, and new spark plugs. Where else should I look.

Cleaning the carb is the next step. Before taking it apart I'd try adding some SeaFoam carb cleaner to the gas to see if you cab boil out any junk. Any carb cleaner should work, I just like SeaFoam.