You narrowed it down to a purely electrical problem at least.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

I got the dithering fuel gage problem (began at about the 3/8 level point) a couple of years ago.

On a 1000 mile road trip over mostly twisty two-lane roads. I started out at about half a tank and only filled it to half full at every fuel service. This caused the wiper to work it's way up and down through the bottom half of the winding. I had to make three instead of one fuel stops each way, but it solved the problem, for now at least, as it has not reappeared, but it may well happen again.

It was probably deposit buildup. A similar problem happened to my Cosworth Vega ten years ago- the gage would not read below a quarter, so I pulled the sending unit for a good cleaning with acetone. It needed a 20 year fuel tank inspection, anyway, and the problem was solved.

This may be more of a problem for those of us who typically fill up at about half a tank. The fuel sender wiper hardly ever sees the bottom half of the winding, so deposits slowly build up until you get intermittent contact.

I store my 190 during the summer (my '91 MR2 is my "summer car"), and this year I'm going to fill it with Chevron (with Techron) gasoline before storage. If the problem reoccurs at some point in the future, I'll dump a bottle of Techron in at the pre storage fuel fill.

Duke