My 2003 Protege LX 5speed 105k km is getting engine code p0300 random misfire. If I run the car from cold, get into 5th gear and drive steady 3000 rpm for 15km it stalls like someone pulled a plug wire for 5 seconds, engine light comes on then car is ok. It is the only time it will do it. If I go up and down through the gears or let it warm up completely before driving, it is fine no P0300. I have read quite a lot of threads here and elsewhere regarding code P0300. I have replaced EVERYTHING. Coils, plugs, wires, EGR, front and back CAT sensors, rear CAT. run premium fuel. To no avail. This car is about to star in a youtube pyro video. HELP.

 

I kind of think that it is electrical. I would wiggle check all wire harnesses I can get to while idling. If you can produce the miss that way you have a suspect.
Disconnect, inspect, clean and reconnect anything. Don't forget the ground connections.

Good luck to you. I know it is frustrating.