fuel pressure dropping over time

Daves69 said
Some gauges do this if they are not vented to atmosphere.
sum-800199.pdf (summitracing.com)

The gauge I have isn't liquid filled, and it is mounted in the cab

(With an electronic sender, im not running raw fuel in there)

mbaird said
Pump is failing when it heats up .

And falling back in line when it cools down?

What would cause that (other then heat, of course)
I mean, what are the mechanics of that issue?