In reply to MM6ADR:
I put “FT817 flashing display” into Google and it came up with this
http://forums.qrz.com/archive/index.php/t-118503.html
from 2007. From the description there, I assume you mean the backlight is flashing rather than the digits etc.
As they note in the comments there, you get “TX error” if you try to transmit out of band with an unmodified rig.
W1GUH found a few power cycles fixed it.
Sorry, I’ve never seen this with mine.
73
John GM8OTI