1998 Mercury Sable Car shuts off without warning and start

Electrical problem
1998 Mercury Sable 6 cyl Front Wheel Drive Automatic 125.00 miles

I have been having this problem with my wifes 1998 24V dohc sable for the past 16 months. It runs great then without warning shuts down as if someone flipped a switch or it will idle @ 300 rpm no matter what throttle position you give it then shuts off. When you try to start it right away, it won't start and just cranks over. Now if you let it sit for like 25 minutes, it starts up and will run for 1 2 or 3 monts till it happens again without any repairs. This has happened over 10 times in the past year and a half. As of late the problem is more frequent. I cranked it over with a screw driver in the sparkplug wire end and NO spark 15 minutes later I cranked it over and I had spark and it started right up. Is it computer related. It is not showing a check engine light.