Having changed the crystal, and after about a month having --:-- returned, I would like to propose a different possible solution to the problem.
The flat white cable from the main board to the front panel board has soldered connectors at both ends. However the cable itself is only a push fit into the connectors (at both ends) with non gold contacts.
It could be that after a couple of years from new, the connectors oxidise and become intermittent. Taking the machine apart, such as to change the crystal, could move the cable enough to make it work for a short period, making you think you have fixed the problem.
I removed my white cable from the machine and carefully rubbed the cable ends with the back edge of a knife to try to remove any oxidisation. I am not sure this will work but I am hoping this will fix the problem for years instead of months.
