quattroforme

Hi Guys. I have had erratic revs after filling up. After putting gas, and moving forward the car will rev to about 1500 then down to 500, up again, down. I run a bit, come to a stop and it does it again. After driving for a bit, goes away and everything is fine. Has happened at the last 2 fill-ups about 1 month from each other. Any ideas? I have also noticed that the gas guage isnt reading correctly though I just replaced the fuel leveler about 5 months ago. It doesnt go beyond half full but I filled her up completely. sometimes also, it drops all the way down before correcting again.
Any ideas would be appreciated. Thanks. Marco

4Driver4

Any codes?