Re the fluctuation... check all electrical connections, including the grounds. I'd disconnect any ring terminals, file them clean, as well as their mating surfaces, and reinstall.

Re the not registering full... When I put a new sending unit in my CJ-7, it would only read 3/4 full when the tank was actually full. After about 6 or 8 weeks, it gradually started reading full. I think the float has to become a little fuel-logged if its new.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.