Jocelyn, sometimes it's the most simple things.... Have you tried loosening the gas cap??? Perhaps the vent hose to your gas tank is pinched? That would allow it to start fine, warm up, but then not be able to supply enough gas and run on lean/less cylinders. Maybe have your mechanic fit a pressure gauge in your fuel line, just a thought... Also, with ethanol gas being used, some have experienced rotted fuel lines in the tank? Valves.... very unlikely! Have you/mechanic done a compression test when warm/trouble is happening? Might show a variance if indeed the valve adjustment was the problem... motor would have to be hot enough to pull plugs and install tester before cooling.