Q: Engine shut off while driving

I have a 95 buick lesabre that is shutting off while driving. I have a 30 min drive to work each day, and at around the 20 min mark, the engine shutoff. No sputter, no noise, just a clean shutoff. I was not accelerating when it happened. I was on a busy road, and was able to shift to neutral, turn off the key, restart the engine, and shift back into drive. It happened once on my way to work, and once on the way home.A few days later, I attempted to take the car to firestone, appx 5 min from my house. The car did the exact same shutoff three times in that five min. Firestone wanted $100 just for a diagnostic, so i have brought the car back home w/out a diagnostic. I'm new to working on my own vehicles, but as a professional aircraft mechanic, i am fairly mechanically proficient. I'm looking for advice on the most budget-friendly way to go about diagnosing and rectifying this problem. I'd also appreciate suggestions for most likely culprits at this point. Thanks in advance for your help!

My car has 90000 miles.
My car has an automatic transmission.