Do you mean that the gauge reads full then 3/4 then nothing? If you do, mine sometimes (most times) does the same thing. Fill up with petrol, ride about 10-15 miles, nothing on the display. Stop for a fag, turn back on again, bingo - gauge reads 3/4 full. Repeat until tank empty!
City Slicker pointed to a bad connection under the tank, but I've checked and re-made most of the connectors (and those under the R/H side panel) without success.
Sorry I can't be of further help.