Dirty battery terminals/ bad cable ends.
I'm having the same issue with my 94. Last year truck died on me, I popped the hood, wiggled one of the cables and it started. Cleaned everything real nice and planned on changing the cables (never did).
Truck was fine through the winter/ snow and ice. First 70 degree day this year, I made a trash run, hit a bump and it shut right down. Lights were dim, buzzer slack. Twisted a cable and it almost started. Cleaned terminals and got it started.
Theory I have is that higher temps mean more resistance(and more corrosion) and make for a poor ground.