My 03 GT has been having the same problem for a couple of years now. It will sometimes just have no power at all. Than some days it runs great. I thought the cool air was helping it run better. But it runs bad than too. I have changed all the plugs out. It has a new IAC valve and new mass air flow sensor. It did have a nitrous kit installed but was having the problem before being installed. I thought I had it fixed at the time. It sounds like it has a miss but all the plugs are new and correctly gaped. Could this be a bad coil pack? The oil looks clean I change it every three months and it is never looked milky. It also burns rich. I am really lost at this point and don't want to just keep throwing parts at it.