i dont think its your cluster,

have you taken the cluster out b4 and yanked around the gauges? e.g. placing illuminious face gauges and had to remove the fuel gauges needle? if so then you will need to calibrate the needle, this is best done if u have filled up on a full tank or when your running on E empty. in this case you will need to turn on the key to ignition (dont start the bastard) and watch the fuel gauge needle, then pull out the needle and put it back in to where it should be (e.g. full tank or empty)

Now if you did not remove the cluster b4, then you will need to remove your fuel pump and check the fuel gauge sensor which should be part of the fuel pump assembly.

check to see if the joints and soldering are all ok, WARNING do not smoke or lit any spark or fire when holding the fuel pump. if the connection looks bad, then let the sensor dry up and make sure u are away from the car, then re-solder the connections. OTHERWISE borrow a fuel pump - similar to the one u got and plug and play it, and see if it still plays around with the fuel gauge. if it doesnt then problem solved, u need a new fuel gauge sensor. BUT if the gauge is still playin, then you will need to do further isolation tests.

E.g. Try another cluster from the same make/model of another car. - there are other ways of troubleshooting but this will involve a multimeter and the EK1 service manual book. - you can do a search for the book and it has steps of troubleshooting a faulty gauge.

try the above and see how u go.

good luck!