Purchased 02 GT

[Hammer]

Check your Computer for codes and take a look at your old plugs when you replace them.

Here's some good info on how to read your plugs:
http://www.densoiridium.com/howtoreadusedplugs.php

Suspected systems are ignition, fuel, Emissions, and finally electronic (computer).

I knew my 98 had a bad computer when I was on the highway one day and the car decided to go into anti-theft mode and kill the engine right when I was decelerating for the exit.

Be sure to check for vacuum leaks at the intake manifold or vacuum hoses.

Check to make sure the following emissions system sensors are good:
MAF, IAT, DPFE, EGR valve and vacuum regulator.

Thats all I can think of right now...