p0300, p0301, p0303, and p0305 (left bank of V6)

Question

Codes appear after about 5 consecutive miles of driving 55mph or faster. Never threw a code at non-highway speed. Pending codes do not turn into reported codes until after a complete cycling of the ignition key followed by highway driving.

Reported Codes start with p0300 first then as you drive p0301 or p0303, or p0305 appears - the other remaining codes soon follow. See this pattern with pending codes also. No noticeable issues while driving except the "Check Engine" light comes on.

New stuff added to try to fix the problem: Complete exhaust system from exhaust manifold back including CAT, both O2 sensors, transmission serviced and sensors replaced, throttle cable, fuel injectors, spark plugs (3 different types), wires,  swapped all throttle body components, timing chain, distributor, cam sensor (3X), Crank sensor (4x), ECU/ECM, Fuel vapor canister control valve. Stamped OEM Intake manifold pan replaced with aluminum billet pan made by Hughes.

 
Compression on all cylinders 150 ±4 psig. Fuel Pressure 50±2 psig.

Added Vacuum gage: idles at 17mmHg when in drive stopped 15mmHg. No noticed drop in vacuum when CEL illuminates.

Answer 1

Driving above 55mph then then the misfre condition occurs, Valve opening and closing , and exhaust back pressure , Would be only component that would cause the problem, Even though the engine has roller lifter and the valve lash is flushed , a broken valve spring can cause this problem , but you would  be able to hear a valve tapping noise, If there is valve tapping noise that should be looked into