I have just had what seems like a similar issue and my theory, which @Maddog75 could potentially disprove, is that it's related to the GSX intgration. It seems to intetionally flash when there is an action you need to do on the "Ground" tab for GSX, but it's neither clear what that action is or if there really is one to do.
Having set up the flight with it enabled in the "Ground" tab, it was flashing on both the captains and FOs OIS well into cruise. Toggling "Enable GSX Control" on/off and then dismissing the GSX menu finally got it to stop flashing. I had to do this separaretly on each OIS, but thankfully it worked as I've got another 12 hours of flying.
