SarTech:
My lifters are not making any noise either... just a theory...
My mileage seems to be fairly consistent, although ambient temps really seem to have a large effect on the mileage. Since I have 3.73 gears (OEM was 4.11) in my X5 will get about 23 MPG in the summer, and about 20 MPG in the winter.
My misfire issue occurs under very little throttle while cruising on the highway. I've been getting very good at finding a good spot while rolling to push in the clutch, shut off the ignition, restart, and continue rolling.
Only issue is a night on the highway - all the lights go out when turning off the engine!!
FWIW - I got rid of a Yukon XL for the X5, simply since I didn't need the space or the lower MPG's. But the Yukon was always good to me, never had major issues (except for having to replace all the brake lines due to the magnesium chloride they started spraying on the roads in winter here).