The ignition key barrel consists of two parts. The locking part which the key operates, and then an electrical switch which is operated by turning the key. It is this switch which can cause electrical problems if it is intermittent. When I suggested you checked this unit, I was thinking of older cars, where the access to the switch part was quite easy. Looking at my workshop manual it seems like one has to pull off the A3 steering wheel to gain access....!!!! Perhaps another member can suggest a good way to get to the switch and check if it is working OK. One quick test is to jiggle the key a bit and see if the cluster illuminates. If it does, it points to an intermittent connection in the switch or plug.