Will it run fine for awhile and then shut off or does it shut off immediately after engaging the deck?
Also, how soon after it shuts off have you checked the spark and found it not firing? Also, does it seem that after it sits for awhile it will start right back up and run fine for awhile?
As an aside, I wouldn't recommend removing a spark plug from a hot engine. It's better to have an igntion tester or another spark plug handy to test the spark.