I gave a printed copy of your post to my friend who specializes in motor control, and he thinks you've hit the nail squarely on the head. The case he was dealing with involved a ski lift operator who had just added surge control capacitors, and these were causing ringing, the peaks of which exceeded the max voltage of the thyristors which was causing premature burnout(the ringing was also causing other thyristors to fire when they should be doing so).