This was the issue on my daughter's '90 a few weeks back. It had been running a bit erratically, had died on her a few times, it all seemed good whenever I looked at it though.

Eventually it dies and wouldn't restart, I told her to leave it there and not mess with it any more.

Went and (thankfully) it wouldn't start. tried to verify the fuel pump was working by jumping the two fuse box terminals, and got something like 1.5 volts at the wire under the back seat. Which isn't right. Took out the kick panel, took a look at the main relay. Noticed it was buzzing. Bad relay? I checked where the 12V+ was supposed to be (thick red wire) and it was 1.5V there as well.

Went under the hood and the white fuse holder seemed fine, followed the red wire from it to the battery terminal - and found the bolt on the battery terminal it was bolted under was loose. Tightened it, car started right up.
--
'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)