fuel sending unit

It sounds like you may have gotten a bad unit. I talked to a local guy that was having a similar issue. He told me he had to fill the tank and work backwards from there. Taking out 4 gal then set 3/4 tank ect. Didn't make sense to me but it worked for him. Brian is a stand up guy he will help you through it.
Best of luck and let me know how you make out or if I can help.

Tom