[SeniorVerde]

Here's another long shot idea, but one I've seen happen:

Fuel additives, leaded gas substitutes, octane boost- all come in bottles with with foil over the mouth and a plastic disk in the lid. You (or a previous owner) take the cap off, pull the foil, and dump it in the tank.

My Dad had a truck that the previous owner always put in some kind of treatment that came in one of those bottles. One time, the cardboard disk that is usually under the cap stuck to the mouth of the bottle, and into the tank it went.
You could drive for a couple of days, a month, maybe half a year- then every once in a while, that disk would get sucked up against the pickup tube, and make the truck act like it was low on gas. Drove my Dad nuts until he caught a glimpse of the thing floating one time he was filling the truck at the gas station.
Spent the whole evening trying to harpoon it through the filler neck reaching into the behind-cab tank with some kind of hot dog skewer. He eventually got it, and the truck never had a hiccup after that.

If we ever dump any additive in the tank, now we remove the foil all the way, and make sure the cardboard is still under the cap.

Probably not your problem, but stranger things have happened.