Can you get B99 or B100 biodiesel where you are? A tank or two of that would help take care of any sludge or grime building up on the fuel level sender. Just make sure you also have a spare fuel filter in the trunk, as that gunk is likely to wash upstream and clog up a filter. If that is in fact the issue, I bet some bio would take quick care of it.

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI