Re: gas gauge not reliable
Taco OEM fuel gauges are notorious for progressively accelerating their fall as the fuel gets used. They sit on 'Full' for a while, then seem to drop faster and faster. Your description of up-and-down behavior, though, doesn't sound quite so normal.
Don't know about Tacos, but... I seem to recall there was a TSB about 4Runner fuel sender / gauge weirdness during a brief period around 1999 or so....