I have a 1211 which is the same as your 680, I put solid engine mounts in it and I haven't had any issue with bolts loosening and breaking. I would remove the driveshaft and inspect the ends and the centering bushings, if the shaft is worn or the centering bushing has fallen out it will cause the shaft to run out of round which will cause all sorts of vibration. The flex disks just couple the parts together, they don't hold them centered.
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