A little bit of back info I bought a 99 Jimmy and after some fixes and finding a couple of electrical gremlins I got her to run but it was really rough and threw a p0300 random misfire code. So I did a complete tune up plugs, wires , cap, and rotor (all acdelco and the acdelco iridium plugs). Also did the intake manifold gaskets with the premium fel pros. After all that it was still rubbing rough until 1500-2000 rpms then seemed to smooth out and throwing the p0300 code still. That is when I started poking around this site and decided to check the fuel pressure which have me some good info that there was a problem.
Here are the results repeated this test 3 times:
Key on engine off. 60-62 psi
Immediately after pump shuts off 55-56 psi
After ten minutes 40-46 psi
Then I tested lesson by dropping psi down to 10psi and waited ten minutes it only dropped .5- 1 psi (2 was considered good)
According to these results I need to replace the fuel pressure regulator but want to make sure I'm not overlooking anything. When I did this test the upper plenum was off and all of the injectors were out to check for leaks (nothing was visibly leaking)
So my question is did I diagnose this correctly and not just throwing parts at it.

 

The regulator is the problem. You need to replace the entire spider assembly as the regulator is integral to the spider unit. You should upgrade it to the Multiport version anyway.