Howdy Ltlfeat!
While riding home from an InZane years ago my '98 Standard started having symptoms similar to what you describe with your '98 Standard. Once home I started going though everything I could think of, but the issue persisted until I replaced the Cover Set.
Assuming that you have the stock fuel delivery system in place, maybe start by checking the vacuum line off the #6 intake manifold for cracks or leaks. That could keep a perfectly good Cover Set from working properly.
FYI, my '98 Standard gets around 32 mpg on average.
Of course, that's going downhill with a tailwind! 