what kind of mods do you have on the car?
my buddy's car had the same problem and it was the opti.... but it was only when it was wet out... once the car dried out, it would stop... I remember back in the day, the old distributers if they got moisture in them, they would do that also... you could just have a leak or crack in your opti causing that... if it happens when the car dries out, i would check plugs and wires, then go to opti if it doesn't fix it.