My XF also blacks out etc. and has been through phases of very erroneous fuel readouts that seem to self-correct after a bit. I think the reason they black out is because of the button used to toggle between oil/voltage and fuel/temp. This presses direct on the solder track and cracks it. This is only a theory that I haven't checked as I have been too lazy to remove the unit. Since you have the unit out already I would suggest taking a look at this area of the circuitry and if it is cracked - solder it back up and put the original back. I think the guage is dependent on a resistor to calibrate the voltage from the sender, so if you have a unit from a sedan and the wagon has a bigger tank, you may have problems.

sorry to hijack the thred but my 5.0L seems to be more fuel efficent & are there any X seriesFM members in Sydney??
If so come tothe dyno day or a cruise sometime as it would be good to see the X series out in force.