JD 277 power loss

This is a neighbor's mower. It's a 277 with four wheel steering. It will start and run fine for about 35 minutes and then begins to lose power to the point where it will stall. Always starts right up. I know that he has had the Deere dealer look at it with no success. It was bought used. He has changed the fuel filter several times. He has installed new plugs. New air filter. Fuel line flushed. Tank emptied and cleaned. I can't count how much carb cleaner has gone through it. His health isn't good so I went by to do some mowing today. Ran like a dream for 35 minutes then will barely run even with the PTO disengaged. I suspect the problem may be electrical and once it heats up something fails. Any and all input appreciated.
Thanks in advance.

Check fuel tank vent, usually in the cap. It may be blocked. When starting to loose power, loosen cap and see if it continues to run. If so, clean the vent or replace the cap.

I reached the same conclusion and tried it; no affect. I finally started over and through a series of steps determined that 1 of the 2 coils was malfunctioning when hot. All this time the owner was convinced it was fuel related. Off to JD and 70$ later it runs like a new machine. Oh well.

Could be electrical who knows? Usually failing electric components cause the engine to skip or miss and wont start back up until it cools. Is it doing that or running OK other than loosing power?
If it is running OK other than loosing power then the news is probably not good. Overheating or wear or both is causing your problem. Clogged cooling fins (and missing air cowlings) cause overheating. If caught soon enough it might be saved by cleaning but most of the time the damage has been done by the time they looses power to that extent. Other thing is worn or scored cylinder walls and pistons caused by overheating and/or lack or service or high hours. Hopefully this isnt the case but I have seen both owners and dealers dump much $$$ into coils, carbs and find out the engine is toasted.
Another thing that causes similar problems is fuel vapor lock. This usually inst a problem unless fuel lines have been rerouted or large filters have been added under hood or sometimes engines with fuel pumps will run OK until hot then vapor lock. If the problems is much worse when hot out versus cooler temps then suspect vapor lock otherwise that isnt the problem. Also try running it with the hood removed to see if that improves things, if so then suspect vapor lock or overheating.

Another thing to check is to be sure the engine is running on both cylinders. I recently fixed another model of John Deere with a Kawasaki twin that had been to 3 dealers and 2 other experts. The engine would start right up and sounded OK but had little power cold and NONE when it warmed up. I happened to bump the exhaust pipe with my hand just after starting it and found the problem. Start the tractor from cold and let it run for 15 seconds and shut it off then check the temps of the exhaust pipes near the head.

you have coil issue they go out after reach certain temp