Multiple Cylinder Misfire (P0300, 0301, 0304)

Car has no obvious signs of an engine issue (Sounds, smells, mileage, rough idle, hard start).

Check engine light intermittently comes on/flashing while driving, caused by multiple cylinder misfire (always 1 and 4). I've swapped the coil packs, replaced all the spark plugs and all the fuel injectors and the codes stay the same, so the problem seems to be localized at cylinders 1 and 4. Mechanic supposedly checked compression and found nothing wrong, still waiting on documentation.

Misfires always go away in less than 1-2 min. They occur at a much higher frequency on long high-speed downhills in hilly terrain.

Forgot to add, there had been an oil leak/oil leaks in the engine previously, this was fixed.