starving for fuel most likely. start with the fuel filter, then the screen in the tank that is attached to the fuel shutoff, check for a vacuum on the tank when it acts up (if it does have a vacuum I'm pretty sure the fuel cap is the vent), then fuel transfer pump, then in injection pump. This assumes your 231 is a diesel and is setup like older Masseys.
Cancel all that if you are getting black exhaust smoke and change the air filter. If still black smoking then your mower may be bogging the engine down.