Drive with the fuel cap loose.

Don't rule out anything just because "it's been running 20yrs." In diagnostics, everything has to be looked at.

When the car dies, prove spark by removing 1 plug wire, putting a spare spark plug into it, and cranking the engine over. You should (of course) have spark at the plug.

To prove fuel delivery, remove a fuel supply hose from right at the carb(s), and crank the engine over. You should have fuel coming out of the hose.

It's not uncommon for there to be a fuel delivery issue where there's debris in the system, and it gets packed into a filter, or into a screen, and the engine runs out of fuel and dies. Then after a few minutes with the fuel pump not drawing on the system, the debris drifts away from where it's packed in, and the engine will restart... because there's now fuel flow. Then the process repeats, because fuel movement causes that debris to pack into where ever it's getting trapped.

When I purchased my Coupe in early 1992, it was doing just this, and it was being sold because none of the shops in the San Francisco Bay Area that it had been towed to, could figure out what was going on. The seller was tired of being stranded by the car, and then being told by a shop that it fired right up, as soon as it was taken off the tow truck. I actually drove the car home, 230mi, at night, with no problem, but it died and wouldn't restart, a week later. I had tools with me, because I knew the failure was pending. I quickly proved no fuel delivery (loosened the fuel line at the cold start injector, cranked the engine over, and watched for fuel spray), and went back to the fuel tank and found no fuel coming out of the electric fuel pump. I removed the hose from the tank to the pump and had good flow out of the tank. I stuck a screw driver down into the inlet fitting on the pump, and found a 1/2" thick slug of sandy grit was packed into a screen that was down inside that fitting. I dug the debris out of the fitting with the screwdriver, re-attached the fuel hose from the tank, and was back in business. When I got back to home, I removed that fitting, and cleaned it "for real." I've never had an issue since, and have driven the car all over CA, even 8hrs each way, to and from Tahoe.

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