From my experience the gear numbers flash when one of the selection or engagement potentiometers is seeing a value outside of its learned range. In my case it was because the potentiometer had gone a bit wrong, my situ manifested itself first by the 5th gear number flashing, then reverse. It was resolved in the short term by EOL'ing it. But i think the rage allowed was getting progressively worse until I only had 1st and 2nd gear.
From my understanding then, I could be the pots are a bit faulty, i replaced mine with one from a scrapper, and it's not missed a beat since.
I found that i could move the problem around by swapping the pots around. IE a different gear would dissappear. Swapping it with the clutch one pretty much killed the car, and had me resolved it was the pots. You can monitor all the different potentiometer outputs in the sensor section of alfa diag, also listed in the drop down boxes are the ranges acceptable in millimeters (mm) for comparison!
Other issues can arise from corrosion on the wires etc both at the sensor, the multiconnector from the selespeed unit, and the main connection to the selespeed ecu.
Hope thats of some help, at least for diag, if you have some spare pots, i'd swap them all out, run another EOL and see if that stops the flashing lights lol