first thing is the fuel pressure get a pressure gauge and make sure its 55-65PSI. 5.7L has higher pressures than the new 5.3L. also rev engine drive vehicle gauge connected see if it holds high. replace fuel filters @30-40Kmiles... next is the distributor remove cap and check for a rotor wobble. most fail at 175Kmiles. using a scan tool measure the long fuel trims should be less than 10%. also short term. if these read positive 15-25% it means a lean condition . then you check for intake manifold leaks . if no leaks and these still read high @2000 RPM then my guess would be the MAF is bad. MAF is not measuring the correct air flow into the engine. p0300 is what your saying ? injectors bad with a p0300 is not common . usually you get a p0306 code etc for a specific cylinder .also the fuel pressure will not hold key on eng. OFF..................