hey i have had this issue for a little over a month now.came with this issue. i have a miss/random rough idle. when comming back to idle from driving the rps drop way low 400-500rpm it almost stalls and then will come back up.the two codes are for a catalyst efficency below threshold and p300 random multiple misfire.does not miss underload or boost just at idle.new plugs and boots and wires.did throttle plate cleaning and adjust.full 3" back with test pipe.turbosmart recir bov into intake.turbo xs manual boost controler k&n i have no idea is ecu has been reflashed or reprogramed.runs out good just funny at idle.any ideas?is their any to tell if reflashed with mitsu mut or medic equipment?

 

P300 is typical for '03 Evo, I have the same, but Tephra mod's can help you
catalyst efficency below threshold is because you have a test pipe(eliminated cat), it also appears in my '03 Evo...
And guess what? My idle is unstable as well! Why? It's because I have the same K&N intake... no you have two options: either to install your stock intake box or to reflash the ECU. More info here(I used magic search button ):
https://www.evolutionm.net/forums/sh...t=engine+stall