Exhaust getting softer?

I know the A/F ratio will make a noticeable difference in the "lopiness" of the cam. If you are running very rich, it will drown out the choppiness, whereas it seems to cripen up when ran leaner. Do you have an AFPR and fuel pressure gauge?

If it's stock, then I doubt the mixture is the cause of this. I would venture to guess that it is just the temperature fluxuation outside.

In my experience, when it was warm or hot outside, as my car got good and warmed up, it would get louder. I'm sure you know this, but when it gets warmer, you will certainly lose a little power, approx. 10hp for every 30 degrees. When it's cold or cool outside, you've probably noticed it to feel "peppier."

The only other cause I can think of is something I think we all experience. If I drive my car every day, it starts to seem quieter, slower, etc.. Now if I haven't driven it in a week or so, it almost brings back the feeling of the day I bought it.

From what information you've given, I don't think you have anything to worry about, just another dedicated stanger making sure your Stang is running tip top!