Wondering Fuel Guage

[AUSVX]

Hello everyone

When I fill up a full tank of petrol and drive out the petrol station the guage goes up all the way, as it should... but after about 5 minutes the guage wonders off and drops off to about over halfway and then would decide to go all the way back up again and drop down and go back up (it does it slowly).
The most annoying thing is that when I realistically do have less than half a tank left, the guage drops off all the way (below E) and maybe rises back up once during the time that I'm getting towards the end of the tank but the guage's indicator stick thing stays heaps below E in that duration......... until i fill up petrol again.
I've been looking around at the How-Tos and some peoples Lockers and couldn't find any info, I'm guessing this isn't common fault...
Can someone please help me with this issue, has anyone had this problem and what do you think it could be?
Thanks a lot, Jordan.