Probably a bad ECU intermittently getting confused. Had similar but not so often symptoms on a 2.8 Celebrity. My mech had it for almost a week at his shop where they used it as their utility car with diagnostics always attached. Nothing happened. You can't diagnose something unless it happens. It my case it was heat related, as I left it at the shop the first cool day of the fall. The first warm day of the spring it just wouldn't start. New ECU fixed it. You can try swapping in an ECU to eliminate that. Your best bet.
Another one I ran into like that was almost exactly like your problem was with that good straight 6-cyl that Chrysler put in my '74 Dart.
225? For almost half a year it would die unpredictably. But it would start right back up when cranked. It was worse in warm weather or when the engine was hot. I did all the normal diagnostics, fuel pressure, fuel flow, etc. Of course it ran fine when I was diagnosing it. Put a new ECU on it - that was a primitive 20 buck part on that car. I can't tell you how frustrated I was with that car, especially when it died on a hot Easter Sunday on the expressway with my family aboard on the way to a family reunion. We didn't make it and got home in a tow truck and busses. Wouldn't even start with the Minuteman towtruck providing a good boost. Had it towed home from where the Minuteman had left it off the highway to the street in front of the house and went back to looking at it. Figured the only thing left to try was drop the gas tank and see if there was something in there that could float around and stop fuel flow when it felt like it. My head dropped thinking about doing that, and the way my head was shaking and the angle of the bright sun was just right for me to catch a glint behind the head. Primary wire to the coil was against the head, and the insulation gone where it was touching. Heat increases resistance, and a bump could move it a bit too. Half an inch of electrical tape fixed it. The reason the wire could touch the head was I had replaced the valve cover gasket a while back, and forgot to rehang that wire on its clip. But it's been 20 years, and I've forgiven myself.I've had a couple 3.1's - a Corsica and a Lumina now - and their controls aren't particularly complex. Did your mech mention the ECU? A good mech doesn't want to stick you with a 200 buck part unless he's sure it will fix it. But if it's working fine when he runs diagnostics, why replace it? That's the biggest problem with this. Don't think an ECU can be returned for a refund. The mech I used when I had the 2.8 Celebrity was a one-visit mech, and just would never do anything by guesswork. He wouldn't take anything for the week the car had been in the shop. Just said he'd get me later when he fixed something, and he did. I've heard that others pick up ECU's at the boneyard to swap when the ECU is s a possibility. You might try that.
--Vic I'm adding this to the tech group. There's some good mechs there.