Why did you decide to change plugs. Was it running rough?
If it was running smooth and you decided to change the plugs just because they're old, then I don't believe one or more coil pack suddenly went bad.
It's gonna take you some time but do what Jeremy suggested. You may have a bad plug or two. If you suspect a plug is bad, while following Jeremy's procedure, swap it to another cylinder and see if the problem moves with the plug. If it does, you know you have a bad plug.
08 GT 5spd
Spectra CAI, Comp Cam Detroit Rockers, Kooks headers w/catted H-pipe, Revolution Automotive dyno tuned, FB430 race mufflers, 3.55s, JLT catch can, BMR Lca's and relo brackets. Prothane motor mounts and sway bar bushings.
20% tint, smoked markers and tail lights w/sequentials. Clear front signal covers, series 3 chin spoiler.