Please I.D. fuel pressure readings/performance problem

First a brief history. This car was my moms put around town car. When she quit driving it, I used it to go out to where I parked my truck. Its only had about 500-1000 miles put on it in the last 4 years. Other than dealing with the EGR valve getting plugged up, it would run fine. I did put the screened gasket on it and have had a problem with the EGR since.
 
Fast forward to the last 6 months. The truck has ran worse and worse. Really bad when it rained. Also, when I was near empty, it would sometimes die when I went around a corner. It would also often act like it ran out of gas. If I would let it sit for awhile, it would start back up and run,,,although badly. Now Im going to be driving the car full time. I put new plugs, wires, cap, and fuel filter. It''s running a whole lot better. But it has a hesitation in it when I first take off in the morning, and after it sit''s for awhile. It also has a slight miss in it.
 
I checked the fuel pressure today. When I first hit the key it slams to 60. After 2 seconds it goes to 55, then with in 2 min its down to 51. When I start it, it stays at 51. This did this for the first 10 times I tested it. At the moment, with out starting it, but just hitting the key, it will stay at 55, or sometimes 60. If I start it, it runs at 51.
 
So would you guys say this is a fuel pump, or fuel pressure regulator problem. Or even both, or something else. Please advise. Also, I dont think it would be a factor, but just so you know, some SOB stole my cat. I have a temp pipe put it until I can afford to get a new one. But Im hoping thats not part of the problem. I am getting a cod for o2 sensor, but thats the only code. Sorry so long, but I wanted to make sure you had enough info.  Thank''s.
 
 

It''s possible that the FPR may be going out, but given the history of little use, I suggest running a couple of tankfulls of gas through it with a 20 oz. bottle of Techron Concentrate Plus additive in each of the two tankfulls and see if that helps by cleaning out the system.