[EC900]

If the problem fixed itself then I’d suspect loose/tarnished ground OR the bulb filament is broken but not separated completely and only making contact when the bulb is jarred thereby “fixing” itself.
Clean up with a brass brush the the grounds and sockets. Try brand new bulbs if you haven’t already, they do “wear” out over the decades.
In previous posts, some say the flasher module could use a cleaning too.