2001 Mitsubishi Montero Sport P0300 Random Misfire: Highway-Specific Hesitation After Warm-Up

Hello everyone, I'm experiencing a persistent random misfire issue on my 2001 Mitsubishi Montero Sport, diagnosed with OBD-II code P0300. This problem first appeared two years ago while driving on the highway at 75-80 mph and was initially resolved by replacing spark plugs and wires—though the vehicle had nearly 100k miles at the time. The issue returned six months later under identical conditions: highway driving over 70 mph, after the engine has warmed up and run for at least one hour. The misfire only occurs during sustained highway cruising with minimal braking or acceleration. When driving in city traffic around Seattle—frequent stops, off/on flow—the vehicle runs smoothly with no rough idling, starting issues, or stalling. A notable example occurred last year on a 330-mile road trip: the first two-thirds of the journey went fine at speeds between 40-60 mph, including frequent hill climbing and braking. However, during the final hour when speed increased to over 70 mph with no braking, the engine began to hesitate gradually—starting as a subtle trickle, then worsening rapidly—followed by the illumination of the check engine light within minutes. Reducing speed to 60-7 or below immediately resolves the issue. I've taken the vehicle to two Mitsubishi dealerships for diagnostics and reflash services, but no additional faults were found. I’ve also had spark coils replaced, fuel pump serviced, and performed a full fuel injection/throttle body cleaning—yet the P0300 persists. An interesting historical note: when my timing belt was replaced in April 2006 by a local mechanic, a 'dry start' procedure was conducted, resulting in bent intake valves. After that repair, the engine light triggered P0174 and P0171 (rich fuel trim) codes—though O2 sensors were functional. A dealership-level fuel injection/throttle body service resolved this issue permanently. I’m now concerned whether this past valve damage could be contributing to the current random misfire or if it’s a separate, recurring problem. Any insights from owners of 2001 Montero Sport with P0300 codes would be greatly appreciated!