help: random misfire

[NISMO SER]

Hi LMs

recently my car's check engine light has been on and off.  I don't know if
it has anything to do with the HS Cai but I doubt it.  The check engine
light would appeared and disappeared after a couple of days, it's been like
this for a couple of months now.  I checked the car using OTC diagnostic
tool, it showed that there's a random cylinder misfire.  I checked to make
sure there's no water get in from the CAI that I installed.  I just went out
to the car to inflate the tire and realized the check engine light came up
again (it wasn't on half hour ago).  The car seems to run fine and strong
but has bad mpg (just 19mpg).  I've changed plugs, fuel filter, caps and
rotor, o2 sensor but mpg doesn't get any better.  Any advice would be
greatly appreciate.  I have 8.5mm Magnacore Wires (3 years old), will a bad
wires cause this?  Anyone using other brands wires that's good on SR20DE?

thanks
David
98 SER
HS Header, HS Cai, JWT S3, Custom Exhaust etc...