Thats a tough one. It could be a problem with your ignition system (plugs, wires, coil). Increased heat tends to increase resistance. If these components are not in good shape that may cause the problem. It could also be fuel related. Sometines when fuel pumps get warm from use they tend to get weaker if they are worn out. Might not be a bad idea to replace your fuel filter as well. I've never seen this problem before so these are just a few suggsetions. Hope this helps.