First of all thank you so much for your donation.I posted the trouble tree for fuel gauge inop or incorrect reading's they didn't have one for a erratic fuel gauge.I also emailed you a copy so you can read it better and print it.Start with step 2 you will need to get a signal generator which is just a device that your able dial in different ohm's value's that you will hook into the harness to replace the fuel gauge sending unit.The chart show's you the ohm's value's you need and what the fuel gauge should read.If it stay's at the correct reading on the gauge compared to the ohm's input and it's steady then you have a bad fuel sending unit.My guess is you have a bad fuel sending unit the gas eats up the contact's or the contact's wear away over time causing the erratic gauge.Let me know what you find.