When fuel tank filled up and fuel pump cuts off fuel gauge will drop to reserve light and stay there for about 100 miles. Also will loose all distant data. After about 100 miles gauge and data resumes and works until next fill up.
Sounds like there is a bad spot on the fuel sending unit in the full position and other position. Have you checked for codes in the computer yet?If not start there and let me know what you find I need the exact code numbers not code descriptions. Also do you have a multimeter to do some testing?