99 Astro dies then restarts when cool

About a month ago astro died while pulling a small utility trailor on 200 mile trip. Lost speed, died. would crank but not start. had towed to chevy dealer, started in the morning for them, they found nothing wrong. Drove it home (200 miles) was fine. Today it just did it again went on a short trip (it was the hottest day of the year, 90 degrees, got home and shut it off, wouldnt restart until it cooled off. Any ideas?