I see that you mentioned the fuel system has been ruled out but I have heard of a few cases now where the fuel pump intermittenly stops working and...
I would think that as the components wear the valve will not open as far and therefore combustion air will be less which the computer will compensate...
I have had this wonky fuel guage experience on more than one E30 and it seems that somehow the connection to the cluster that connects the fuel guage...
Separate names with a comma.