i have a 94 GTb wagon with similar fuel guage problem, i have replaced/use three sender units. so now when time permits i will replace the gauge with another and check wiriing and earths etc
subaru fuel tanks are a bit ghey in that they have a tunnel/section built into them to allow for the drive train.
This almost splits the tank into two sections, so when the fuel level is low and/or at a certain level.. when you go round a right hand corner the fuel all leaves the drivers side part of the tank where the sender unit is, making the gauge go low.. so that when you go round a tight left hand corner the fuel level fills the drivers side again making the gauge go up.
As part of the sender/pump assembly in the tank there is a pick up on the passengers side.. to pick up from the fuel from the "other half" of the tank, this is achieved via vaccumm
Ghey fuel setup is ghey