Try improving the layout of the power components - you are usingf decent busbars for the DC-link arn't you?
Next have a play around with the gate-drive. If you can slow it down a bit (at turn-off) you will ok increase the switching losses of the thyristor but you will help it. - do not slow it down loads it still needs to be as fast as possible, just take the edge off it
with an improved power layout and a slightly slower turn-off then start looking at a RCD snubber circuit.