Small Engines (Lawn Mowers, etc.): Mower dies out and wont restart., yardman, fuel cap

Question
The mower is a couple yrs old its a 6.25hp yardman with a briggs&strat engine.
The mower starts up fine and runs good for about 15-20 mins then it like "coughs" and dies out and won't restart.  Then if we leave it sit for a while it will start up again and die again after a few mins. It seems like it dies out from overheating or something, I dont know. I have changed the sparkplug and put new oil in it, so what else should I do?  

Answer
It's possible that incorrect tappet clearance and/or burnt valves can be causing it to run poorly when it gets hot. Another issue related to time can be the fuel tank vent. Try loosening the fuel cap when the engine starts to die.  Yet another issue can be that the ignition module is failing when it gets hot, in which case you need to check for spark immediately after the engine dies (should jump 1/8" gap).  Hope this helps.