Kid with a truck

I've been reading up but just can't particularly figure it out for my situation.

I have a 00 5.4 with 125xxx miles. It runs great but when sitting between and hour and 2.5 hours, it just cranks and sputters to a start. So I hooked up a fuel pressure gauge to it. I have 32 psi at and idle and 40 psi with regulator vacuum disconnected. So I let the truck sit an hour and a half knowing this is when it doesn't like to start. I hooked gauge up and turned key on.....Pressure read 20 psi.

My question is what makes it not pressurize up to 32 psi like it normally does when its just running or cold???

thanks for any advice, I think its a bad regulator???

Kid with a truck

oh and by the way, when it read 20 psi, I cranked it a second or two and looked at the gauge.....it read 40 psi. Then, I went and cranked it again and it sputtered to a start.