"But the fuel gauge needle is always moving and never giving an accurate reading."
I hate to say it, but this is normal. It does not have a conditioner in line so if you go around a turn or up a hill it will move by a quarter tank one way or the other. The accuracy sucks on them too.
Like JimC I have never seen low fuel pressure checked for on a PPI (pre-purchase inspection) and, as you can tell from the posts on this website, D90s are not honda accords and they are 12-15 years old at this point and issues crop up for no reason even on the best maintained trucks.
As far as what the problem could be, if there is one, it could be the pump or it could be the fuel pressure regulator or the vacuum line to it or a clogged fuel filter.
All this being said, I know from indirect experience that a call to Keith and he will provides after sale customer service that is awesome.
*not legal advice*