1999 Chevrolet 2 Door Tahoe, 5

1999 Chevrolet 2 Door Tahoe, 5.7L, 4x4, VIN R, 122,200 miles
Original problem: Service engine soon light & Code P0300 only under heavy load. (Random cylinder misfire) Light would go out next day. Vehicle drove fine under normal driving circumstances.
1) Changed plugs, wires, rotor and distributor cap. Had timing and distributor set to proper settings
2) Changed inline fuel filter. Still thru code P0300
3) Installed new AC Delco spider injector kit. New problem: Missing (Stutter’s & Jolts @ varies RPM’s) Still thru code P0300.
4) Installed Delco reman/distributor and new Delco Platinum plugs. No improvement!
5) Fuel pressure test showed pressure drop from 60+ to 42. Installed new GM fuel pump and additive for excess water in tank.
6) No improvement... Throwing too many $ and parts with no results... Fired mechanic! Need Help...