Alex, 2 things come to my mind from your description.
Quite possibly the fuel filter is in need of changing.
It is also possible that the atmospheric vent hole in the fuel tank filler cap is clogged with dirt/debris and if so, a vacuum will form in the tank as the petrol is consumed, and that vacuum will impede the flow of fuel to the carb.
Probe the cap vent hole with a stiff wire and if you have compressed air, blow it out well with air.
You did not say if the labouring/stalling occurs as soon as you try to begin cutting, or if it develops later. "WHEN" it occurs, can point to whether it is filter....or vent related.
If it occurs as soon as you start up the tractor and begin mowing it is probably the fuel filter.
If it begins some minutes after you begin mowing (say...15 to 20 minutes) and seems to get worse the longer you keep mowing, it is most likely a clogged fuel tank vent hole.
Please do update us on your findings.