I had a similar problem. It would sometimes stall while warming up, but would restart before I had time to check anything. I eventually found and repaired a circuit board trace in my fuel pump relay, and it hasn't stalled since.

Since your car takes longer to restart, a simple test would be to remove the fuel pump relay and jumper pins 7 & 8 to power the fuel pump directly. If it then starts and runs, you've found the problem.

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1989 300 SEL that mostly works, but needs TLC