p0300 random misfire cod

[kabman]

Hi, any body have are had this problem2001 GMC sierra 1500 4.3l  , I changed a leaking regulator and installed replaced new injectors on a spider injector  installed new distributor rotor and cap, go to check fuel pressure reads low about 45psi drop to about 40psi  , so installed new fuel pump and running its below 50psi and still running ruff and misfiring. Need help here any body!