aftermarket fuel sending units are known to be inaccurate, and the temperature sending unit may not be getting a good ground due to excess sealing material [tape or dope] on it's pipe thread, or the ohm value may not be the same as the factory original one.
old wiring could also be an issue with too much resistance, bulkhead connections [in the case of the temperature sending unit], or the connections to the sender units themselves could be not making complete contact due to corrosion or misshapen ends.
just a few things to check. i hope this points you in some direction to find the problem[s].
please keep us informed as to what you find.
