Gas Gauge

[96C280]

My gas gauge has recently been acting erratically. It only happens after I fill up the tank. The gauge will show full then fall to empty, sit there for a while, then jump back to full. This will continue until I have driven 20-30 miles (~1-1.5 gallons?). Aftter that it will act normally until the next fill up. I had the fuel sender changed under warranty years ago for the same erratic behavior. Probably not going to change since I'll have to pay the bill and not dangerous (I think). Just wonder if anyone knows the inner workings of the gauge and how having a full tank of gas causes this behavior. Thanks.