If it was a bad ground then wouldn't it happen at more random times? Not just when it is below 1/4 tank? I am guessing sender, but I don't have knowledge of that specific car. I searched and found a thread "300CE fuel tank problems" but it is a little unclear what it is talking about. Might help you.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra