Only just seen this thread. Apologies for resurrecting.
This is exactly what's happening with mine. If the gauge reads around 1/8th and range says 79 miles, it'll cut out. I've had it die on me twice around this point (during the first month of ownership) - I keep it half full now and have yet to get into the tank and carry out a repair.
Interestingly, when I fill it, the gauge never reads full.
Our Iveco Daily vans at work had exactly the same problem. 02 Vogue