Hrrrmm....strange one. It could be one of those stupid EGR valves on the intake manifold, that can cause the motor to 'hunt' at idle, especially on the XF EFi's. Sounds like whoever did the rebuild didnt re-time it properly or check for vacuum leaks. You will probably find there is a vacuum leak somewhere causing the wierd running probs.
As for fuel consumption, hell, you're doing good if you can get 500k's to an 80L tank on one of those, the most I ever got out of mine (XF EFi 5spd) was about 400!! with moderate caning of course. My XF panno that I had used to drink even more, with a rebuilt motor, it was carbed though, with a three-in-the-tree speed.
There are only 2 states to be in:
Western Australia, and pissed.