What may be causing lawn tractor to sputter and die during operation?

Runs fine for 15 minutes, then sputters and then dies. Does not restart unless I wait about 10 minutes, then will run fine for another 5 minutes before sputtering and dying again. I have 17 hp Briggs & Stratton engine. Have changed spark plug (old plug looked clean), air filter, fuel filter, and added fuel stabilizer. Someone on Yahoo answers suggested unplugging vent on gas cap and/or running with loose gas cap -- tried this but same result.