1992 Buick Lesabre 20 Minute Buick

This 1992 Buick of mine has developed this issue on the last 3 months.I have owned it for 7 years. It will run 20 minutes. After that it will stall. Or you can shut it off. Either way it will not restart for anywhere from 1/2 to and hour or more. It appears to have no spark when this event occurs. Things I have done: Changed water pump. Cause the after market one failed and was causing over heating. Replaced the bypass hose and cleared the sand out of the fitting. Replaced the IAC valve. It was a solid plug of carbon when I took it out. Tested the ignition module. Tests good but we did not test it for 20 minutes and heat it up. Repaired several ground connections. Redid power connections at the fuse block under the hood. Changed all the vacuum hoses. They were cracked. For all that effort. We have strong running engine. But it still shuts off in 20 minutes or so. I do know the magnet has went AWOL for the cam position sensor. But it has run this way for 2 years. Last time I drove it. It was quite cold out (in the 20's). And I had a check oil light. Crankcase has plenty of oil. Nothing knocking. Restarting it cleared it. Has not re occurred. The 20 minutes is odd as it is not affected by weather. Hot, cold, wet or dry. It still goes away. Any ideas.