Attention: all automotive experts.
FYI - My problem child, a 1999 Pontiac Grand Am SE (2.4 L with 4 spd auto trans).
The problem/fault has been solved... The culprit is a plugged Catalytic Converter causing excessive back pressure resulting in the engine's exhaust gases not being properly vented from the cylinders, which in turn is causing the HO2 sensors to have false readings of a 'lean condition'. Overall this fault is causing the ECM to over fuel the engine.
How was it found?
After exhausting all the diagnostic procedures associated with the DTC P0300 to be set, which included the entire fuel system, and ignition system. I removed the Catalytic Converter from the vehicle and...
Low and behold, the engine responded with a roar! (I remembered to wear my hearing protection, but it was loud enough to be heard in the shop office.)
2.4 Hrs later, the vehicle was fitted with a generic replacement catalytic converter, that had to be welded into place, and the absolutely joyful customer was merrily settling his invoice and headed towards a vacancy withing a stream of traffic.
This particular fault had 2journeyman mechanics stumped for a week... I received the vehicle from them and spent a total of just over 20 hrs in diagnostics.
Something to watch for in the future,