More often than not lately I find trash, dirt and grass clippings that have made their way into the fuel tank. This stuff gets sucked into the pickup tube and fuel lines way before it reaches the fuel filter.
Once the mower shuts down, this stuff relaxes inside the lines and not get compacted until the next time the machine is ran for a period of time.
Some will run 15 minutes; others will run for an hour until they eventually shut down.
The other suspect is the ignition switch. They are cheaply made and will shut a mower down when they get hot.
Drilling a hole in your fuel cap has now introduced the possibility of water and fine dust getting into the fuel tank. Modern fuel tanks on riders have a vent valve on the top of them with a hose attached that goes to the air cleaner box.
"Vapor locking" is a problem with fuel boiling inside the carburetor float bowl due to an overheating engine.
If you have not done it, remove the upper engine cover and clean out the area and the cylinder cooling fins. Rodents like to make winter nests in hidden places under the engine top cover.
