Low fuel pressure

My XJ started running poorly 2 weeks ago. It started as hard starting and progressed into rough idle and sputtering at lower rpms/speeds. If I scream it it seems to run ok but cruising it starts sputtering. Did a tune up and replaced fuel filter. The starting and sputtering improved slightly but not much. I got a fuel pressure testing and hooked it up. The manual says it should register around 39 psi with the fuel pressure regulator vaccum fitting disconnected and 8-10 psi connected - the pressure guage says around 20 psi either way, maybe 1 psi less when vaccum fitting reconnected. Anybody got any ideas?

I do believe that is 39 and 30. If you are reading 20 with a new filter it is time for a new fuel pump. If you have 4.0 recommend Bosch 69302.

I agree with joe about the gage reading showing a pump issue, but I'm curious about a belief I've had for a while. I'm very curious, if anyone has an answer to this question, assuming bad fuel pump / supply: Why would a motor run good when "screaming" (high fuel consumpiton time) and sputter when "cruising" (economy mode)? I've always figured the test of running up speed at full throttle, would eliminate the fuel pump.

I had the same problum it was the line INSIDE the tank from the pump to the metel tube that runs out of the sender. the line comes loose and slides off the tube when low fuel consumpiton. when ran hard slides back on cuse its using more fuel.
low fuel rail pressure
the gf jeep started running bad and I checked the TPS changed the filter I thought it was the pump so I got ready to replace it, pulled the sender out of the tank and found the line was almost all the way off the sender.

Want to close this out by saying that I got a Bosch fuel pump from Advanced Discount Auto Parts - about $95-100 with tax and pick-up screen. I didn't have all my normal stuff of being at home with time on my hands - I was working on the road, staying at a hotel. I also couldn't get my fuel level down below 1/4 tank so I parked the front tires up on a curb where the parking lot sloped down away from the curb. This gave me extra room to crawl under my XJ and let the fuel in the tank flow to the back of the tank - it worked. Had to fish out the rubber stopper from the tank and trim the top of the colar that comes with the Bosch pump to fit in the stock bracket but after that everything went together and worked great. Thanks everyone.