Ok I hav a 01 dodge 2500. I hav a live line fuel pressure gauge. The other day I saw it drop to 3 psi at idle then shoot back up to 12psi. I still hav the stock block mounted lift pump.wen it drops down to 3 psi and u goose the throttle it'll jump back up to 12psi. But sometimes u hav to shut the truck off. I'm stumped on wat it could be.

 

If it was me I'd pop the hood and bump the starter and listen for irregularities when/if pressure drops. Maybe replace it if it cuts out or runs like crap and looses pressure. If you have a stock lift pump still that sucker needs to go, they are notorious for going bad. The stock pumps were designed to push fuel not pull it and they are mounted on the side of the block. I could run my stocker down to single digits, now i cant get my FASS below 14psi. Just my .02. Not an expert mechanic or anything just a backyard mechanic who happens to be diesel enthusiast too.

 

I cant get my FASS below 17psi. Sounds like you LP is packing its bags and ready to leave. I would address this before you end up with a dead VP on your hands.