Sounds like your fuel level sending unit is on it's way out - others have reported it happening. See here:
Gas guage flashing
Anybody have an issue with the gas guage on the dash cluster flashing and going to empty (even after I just filled up to full)? It's intermittent and goes back to normal after driving for a while. I've also noticed that the gas guage doesn't show the correct amount of fuel when it is...
Fuel Level Sender
Hi guys, My fuel gauge goes from full to zero randomly. I suspect it is the fuel level sender. Can anyone point me to any DIY links or do I have to bring it into a shop or a good repair manual?