red82gt,
I've been reading that some Fords use an "Anti-Slosh-Module." I'm not sure if Mustangs do? If so, it's supposed to be mounted behing the instrument cluster.
I guess it makes the fuel guage react slowly, so as the gas sloshes around, the needle doesn't move up and down. Also, the article claims that in some applications the anti-slosh-module is calibrated to the fuel gauge and they both need to be replaced as a single unit! Again, I don't know it this is true about our Mustangs.
(if you want to read the article I mentioned go to:
http://www.smpcorp.com/download/view/TT3Q02.PDF
It's a .pdf file, look at the second page, second column)
It seems tough to get accurate information on this subject; probably becuase it's a rare problem that doesn't come up that often! Maybe I'll call the dealer today but my experience with them in the past is that they don't offer much assistance on a 14 year-old car!
Phil