I got a evo 9 with mileage of 50000, and I am suffering from a persistent check engine light. The diagnostic is always a random multiple cylinder misfire. I carried out a 60K maintainence at P-tuning 2 weeks ago. They changed the NGK spark plugs for me and said everything else on the car is fine. I thought this could solve the misfire problem but apparently it didn't as the light went on after just about 1 hour on my way back. Checked again, still the same problem. I really dont know what happened. Also, there is a constant idle rpm drop on my car too. It happens randomly, with my idle rpm drops to only 500, and there is even a couple of times it stalled the vehicle.

Any suggestions about what happened? Thanks!!