it has nothing to do with HDMI.
I stand corrected, for some reason I just assumed it was HDMI connection, As these where a problem in the past
But as I would suggest it could be a bad connection with the XLR, be it the plug / cable or the XLR board.
As the OP suggests it would be a simple operation to disconnect all the XLR connections and just use the RCA one at a time.
