lack of power/stuttering

Hello all, I have a 1994 GT (5.0) convertible that has a overall lack of power and it runs rough in wet weather. It is mostly stock but it does have a Mallory aftermarket ignition system. I pulled over after running about 20 miles and notices that my headers were glowing, not brightly, but glowing. I was told this was because of a rich condition. Is this true and what should I be looking for? I recently replaced the fuel filter, it helped, but it still is running like we're pulling a trailer.

lack of power can be caused by many things. Like you said, fuel filter is one. The fuel pump pressure could be low. spark plugs could be the cause, exhaust system restriction, retarded cam or ignition timing, low compression, or accelerator not opening throttle all the way.
Do you hear a knock? if so the computer is retarding the ignition timing to reduce or eliminate the spark knock.
But to start, I would check the spark plugs and the fuel pump, and the throttle.

Well i've narrowed it down a little, it runs bad when it's humid or raining outside.