Tank sender unit is hosed, I'd bet. Maybe as simple as a cruddy
connector or bad ground, maybe as complex as needing a new sender unit.

My Mazda 626 (an '82) had a "psycho gas gauge" for a while, too -
Finally got around to pulling out the sender unit and found that the
resistance wire that the float's slider moved on had broken in multiple
places, , shifted around a bit, and landed so that things were sorta
"crosswise", electrically speaking - In some sections, the reading was
reasonable. In others, moving a tiny fraction of an inch would jump
between segments of the resistance coil, causing *HUGE* changes in the
level that the gauge "saw". Still other segments had no coil at all, so
moving from one of the "sane" segments onto the "bare spot" would appear
to go from, for instance, half a tank to dead empty in a matter of
seconds, or from dead empty to 3/4 tank, etc.