2001 Montero Sport, P0300 Random Misfire Code issue

Greetings, I've been unable to locate a random misfire problem on my 2001 Mitsubishi Montero Sport. This first occured two years ago when driving on the highway at 75-80 mph, and was diagnosed then as misfire, replace plugs and wires. Mileage was nearly 100k, so I thought maybe that's the answer and had them replaced. Well, it happened again 6 months later under the same driving conditions of on the highway at 75-80 mph, and this time the test revealed random misfire, p0300. While this has been a consistent problem, it only duplicates itself driving on the highway over 70 mph, after the engine has warmed up and been running for at least an hour. Not exactly something my local mechanic can test for......ARGH!!! Driving around the greater Seattle area where I live on a day to day basis in off/on traffic, it's smooth sailing. No starting problems, rough idling, engine dying..nothing! Another example of this occurred last year with a 6 hr drive, about 330 miles. The first 2/3 of the way, there were no issues, driving between 40-60 on smaller two lanes roads, and a good deal of breaking for hills, mtn passes. Only when the car was allowed to open up in the last hour with no braking at 70 plus did the misfire occur. The best way I can describe the hesitation is it comes on at first like a trickle, very slight, then becomes increasing worse, with the engine light coming on shortly after that. This all occurs within a couple of minutes. If speed is reduced to 60-70, the hesitation stops. I have had the car in to a couple of Mitsubishi dealerships, and beyond running some diagnostic tests and doing a reflash, nothing else is discovered. I have had the coils swapped and replaced the fuel pump, but still no luck. All this being said, there is a wildcard issue that I hope is not part of this problem. When my timing belt was replaced in April 2006 by my local mechanic, a "dry start" was done. He tore the engine down to expose the damage and replaced the bent intake valves. Two weeks after this, the engine light came on, reading p0174 and p0171 codes. The fuel trims were maxed, but O2 sensors were OK, and after a fuel injection/throttle body service (dealership term), this has never happened again. I'm at a loss here and could really use some help!