P0300 Fault on 2001 Chevrolet Suburban 2500 6.0L - Check Engine Light Flashes Above 73 MPH

I am experiencing a persistent P0300 misfire code on my 2001 Chevrolet Suburban 2500 with a 6.0-liter engine. The truck runs smoothly under normal conditions until it reaches 73 mph. After approximately 30 seconds, the check engine light begins to flash and remains active until I reduce speed back to around 72 mph. Despite having already replaced the spark plugs and wires, the issue persists. The vehicle has accumulated 107,000 miles and performs well in acceleration beyond 73 mph—only the check engine light is affected. It was previously a Michigan fleet vehicle, so I'm wondering if there might be a speed-based programming restriction or calibration issue. Any insight into potential causes such as fuel delivery, injector performance, vacuum leaks, EGR operation, or the need for a compression test would be greatly appreciated.