Small Engines (Lawn Mowers, etc.): murray lawn shuts off after it gets warm, vapor lock, fuel cap

Question
I have a Murray 17.5 hp riding lawn mower(Model 42508x92a).  I have replaced the spark plug already and checked the fuel filter.  However after about 5 minutes of cutting it shuts off and will not start.  The engine will turn but will not start.  After it sits for a while(3 - 4 hours) then it will crank again.  It has a Tecumseh motor OV175.  Any ideas as to what the problem may be?

Answer
Could be a few different things, bad coil, causing the windings to act up when they are hot and have expanded. Fuel line vapor locking. Need to check if it's getting fuel to the carb when it won't start, as well as spark to determine what system is failing. Could be the fuel cap not venting correctly, causing a vapor lock. Could be a engine in need of rebuild losing it's compression when hot due to internal parts, and expansion when hot...  Tom