I have the same exact problem with my '90 'Stang.
I'm going to be changing out the fuel sending unit (I need to get at my fuel pump anyways, so....). After I do it, I can tell you if it worked for me, or not.
I'll probably end up unplugging the gauge in the back, but that'll be after I change out the Unit.
If that doesn't fix the problem, then I know it's the gauge itself.