Just spoke with Apple support again and she mentioned a step no other support advisor mentioned...reset the NV RAM. To do this, I held the Option, Command, P and R keys while starting the computer. I held the keys for 20 seconds and the computer did another restart, after which I released the keys. This has stopped the flashing for now. If the flashing restarts again, I'll add to this post. Thanks for your input, if you had responded to this post.