+1 on the seafoam. excellent stuff. but as stated above, research how to use it in the intake before trying.
if you dont want to attempt it, put the whole bottle into the tank.
run the tank all the way down and burn about 1/4 of that tank.
also, you may not be driving the car long enough to get the cat and the engine hot enough. i had a problem with an old hatch of mine, put a new cat on and everything, wouldnt pass.
drove on the interstate (regular driving at the speed limit is sufficient, no hot rodding it) for about 20 to 30 min then went straight to the emissions place, passed no problem.
if you try to hot rod it (ie gunning it to get it hot faster) all you are going to do is dump unburned fuel into the exhaust, giving you the same issue.
keep the foot on the gas pedal light.