If the problem is a drop of water in the bottom of the carburetor float bowl touching the water sensor, it may only cut out when you are lurching around mowing grass - it probably wouldn't happen while the machine is parked with the engine running unless there was quite a lot of water in there.
Most likely the cutting out is caused by the engine stop device getting grounded intermittently. There could be various ways that could happen, including the carburetor water sensor but also including safety devices intended to keep you from hurting yourself. Does it still cut out when you actually cut grass? If it does, will it still cut out if you temporarily bypass the carburetor water sensor? I think Bruce's diagnosis is very plausible - the symptoms you described sound just right for a small puddle of water bouncing around in the bottom of the float bowl and sometimes landing on the water sensor. However it could be anything that occasionally grounds that kill wire when the mower bounces over the turf.
