Mike,
First off, 200 miles on a third of a tank! Wow, I would love to get that much from a FULL tank.
On my GTD the gauges started to stick (I eventually discovered) and wouldn't show more than half full. I took them apart and adjusted the needles (carefully and slightly) and they work like a charm now. Of course, mine are almost 20 year old Smiths which yours probably aren't, but I just thought I'd mention it just in case.
I too started at the sender and found that the resistances at full and empty were as expected, so that led me to the gauges and/or the small voltage stabilizer (which tested fine).
Good luck resolving it.
Mark