I have a big line kit with the FP by the tank on the frame. I have an Airtex lift pump with the appropriate fittings, also an 80 micron mesh filter before the pump. Get about 18.5 psi on start up after the truck is warm, 14-15 cruising, maybe 12.5 towing my 10k fiver on a grade, maybe 10 WOT.
After about 2 hours with the fiver, pressure slowly drops to under 5 psi with the fiver. I turn off the ignition and on, back to normal for 30 mins or so. This repeats until we get where we are going. The main FF was replaced, so that does not seem to be it.
So, my thoughts are:
1. Dirty 80 micron filter
2. Replace Airtex FP (Not likely as it works fine most of the time?)
3. ECM going bad?
4. Other (Perhaps Space Aliens)?
FWIW, it is stock with gauges, 90 gallon bed fuel tank used to top off main tank, Big Line kit with Airtex FP.
Thoughts???

 

I'm thinkin 1,2 or 4. The other thing I would check is all the wiring and connections, could be gauge, or fuel pump wiring heating up and growing resistance....

Just a thought.

 

I'm thinking #1. I had mine clog and then the spring inside literally blew threw the bottom of my pre-spin on filter. At the time, I couldn't figure out which lousy Ohio fuel pump was clogging all my filters.