fuel delivery problems

i have got a good one, for about 2 years now i have been having problems with the power and the fuel mileage stinks on my 98 chevy/gmc pickup. it is a 5.7 with about 101,000 miles. the truck has no power at all. now its getting worse to the point that at a light or just being parked the truck feels like it will stall but does not. fuel pressure running is 54-55 psi, low but still not low enough to cause this, new filter, new updated spider assem., regulator, new intake gasket(done recently) etc. i tested it about 6 months ago and pressure was the same but no stumbling then. once and a while if i push the truck it will backfire, no CEL's. when truck is shut off the pressure drops slowly, does not hold pressure but no visible or smellable leaks.

get a new fuel pump, that would be my guess. Not only does it sound like it is on the way out, It sounds like the check valve on it does not hold pressure......or look for a leaking injector