Petrol tank indicator

My 03 Focus (1.6 Zetec) runs fine, but yesterday the needle on the tank indicator showed a quarter full, which was correct as I'd just filled it to
a quarter of a tank, but 15 minutes later, it indicated that I was almost empty. Another 15 minutes later and it showed the quarter full mark again. Anyone know why this happens, and is this a problem I should have checked out? TIA.
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this model year had a few problems with the fuel tank sender card (inside the tank)you can get a modified card and float without getting the expensive fuel pump.
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