2000 Subaru Legacy Engine misfire

2000 Subaru Legacy Wagon GT with approx 150,000 miles (5 speed manual). The wife�s car has an intermittent (though becoming much more frequent) high speed miss. The car will start and run fine. When under load such as entering the freeway it sometimes goes flat between 4500 & 5500 rpm and the �check engine light� flashes. Once at cruising speed (75 mph) the stumble will slowly subside, the �check engine light will stop flash (though not go out), and the engine will run fine. This has been happening for the last three months. It will throw four codes; one each for a misfire in cylinder #2, #3, and #4. The fourth code is for the knock sensor. Plugs, wires, and the coil have been replaced (twice). The knock sensor has been replaced and timing checked. Each time the car will run fine for a day, maybe two, then its back to misfiring and the engine light is back on. Thanks in advance. Michael