Fuel pressure regulator?

Can I replace the leaking quick connector from my fuel regulator to my supply line with

I bought this truck last october, so im not to sure on its maintenance history. Since I got it ive replaced the Rad due to sever leak that was probably patched with an additive before the truck was sold since the leak started appearing about a week after I got it, and due to stains that were on the shroud im the same spot that was getting wet when the leak appeared. Other than the Rad change the only other things I did was change the air filter and and a small set of LED driving lights. Since that work was done its been running nicely. The only thing I did notice was that when I come to a stop after driving at almost any speed the RPM's would drop down to near 400 or less then go back to idle then drop once again about half as far then idle fine. It has stalled twice under the same conditions of coming to a stop. Recently though the truck decided to just quit running while I was driving in town. It had been running for at least an hour and a half that day and I had filled the gass tank the day before on my way home. Couldnt get the truck running so I had to leave it in the parking lot I had managed to coast into when the truck died.( Was short on time since I had to work a split shift that night and be back at work in half an hour after the truck died) So the next day I haul the truck to a shop I work at and at a later time me and a mechanic take a look at it. We started off with trying to crank it over a few times no luck other than the odd rough sputter every now and again. We pull the first two spark plugs on each side one at a time as we check for spark, Looks fine. Then we open up the distributor cap and check for corrosion also looks fine but we still gave it a couple scrubs with some fine grit and throw it back together. Crank it over same result. Hooked up the OBD and no codes came up. We at one point hooked a fuel pressure gauge to the rail and turned on the key we got 42psi and when we cranked it the gauge would wave between 35-50psi. So I bought a new fuel pressure regulator drained the tank dropped it put the new regulator into the pump hooking everything up the same as it was and tried to start it again, still no luck. Hooked the gauge back to the rail and turned the key on, jumps to 42psi. Turn the starter and psi stays pretty much the same. The mechanic suggested that my pumps not putting out enough and that I should have just got a new pump assembly anyways. So I did, just installed that today and tried again still the same results, mechanic hooks the gauge to the rail again and were getting 50 psi but once the pump kicks outthe psi steadily drops till I either turn it over or reset the key and turn it on again. At the moment I dont know hat else to do. I called the local dealership and they said check injector pulse(which I dont know how to do and the mechanic thinks it cant be an issue there( for some reason he thinks whenever I mention injectors im saying that they all failed at once and and thats not possible) the only other thing the dealership said it could be wass dirty spark plugs. Im going to buy a new set of spark plugs, and put the new regulator off the old pump onto the new pump also im thinking I might as well do an oil change while its in the shop as well. Couldnt hurt. But at this moment im stumped and starting to have doubts about our mechanic and starting to wonder if anyone else has some opinions or ideas on what my issue is?

Ya it looses almost all pressure in about 6 minutes, no leaks in fuel line or connections. Decided to leave the regulator alone on the pump and got another ordered to replace the current "new pump" since the regulator on it is faulty

V8 four wheel drive automatic 150,000 miles.

Where is the fuel pressure regulator located on my truck listed above it a 1500 with a 5.9 liter 360 engine?