Intermittent not starting?

The first sign of trouble was mid-April, car not starting. Replaced battery as it was 5 years old. Then 3 weeks later, not starting. Towed to auto electrician they had for 3 weeks, started every time except for once. They thought it was an immobilization fault, they thought the keys were maybe the problem. He said he was not sure, maybe 90% key was the issue. I saw the reading, code 12 - starter signal and code 53 Antenna. I didn't think it was the keys, so we drove it again for another few weeks, then it happened again. Towed to a Subaru specialist, where they tested it for 8 weeks, only faulting once, and they said not long enough to get a reading of fault codes. I took it there, as it needed the timing belt done. (I took the risk of getting this costly work done, hoping they could find the fault of the intermittent not starting). I really want this car to keep going for a few more years, as it is so costly to replace cars in Australia, new or used at the moment. I know it is 23 years old, but it only has 190K km. I'm the only owner, for the last 4 years, my 25 year old daughter has been the driver of the car.

Subaru outback Ltd Australia.