If happening when throttle is advanced somewhat (like during tramming or mowing) then my first guess would be a loose drive belt - meaning engine would race AND the mower propulsion / blade power/speed would jerk or drop off /lose power at same time. Ditto if you have a hydraulic transmission and are low on fluid or the hydraulic fluid filter is plugged up, causing air to pass through the hydraulic system. I would guess if this is the case your mower will below on power or fail to climb hills.
Could happen at any throttle setting, but especially if happening at idle throttle setting, I would suspect a loose or broken connection in the throttle or governor system - first thing I would look for would be a broken or disconnected spring allowing the throttle plate on the carb to "float" back and forth. A total disconnection of governor or throttle linkage cable or rods could also do this. If this is what is happening, then when mowing or tramming the speed of the mower/blade would increase and decrease proportionally with the engine speed. The mower power would be relative to the engine speed, not drop off like in the first case.
