I had this exact same issue with my lil axolotl guy's gills freaking out and i was able to fix it by going into settings and increasing the max fps! it was at 12 by default but i set it to 14 and its working as intended :] hope yours gets working for you soon <3
I'm finding it might have been related to a few issues pertaining to bounce force and gravity being different from when it was originally rigged, as well as the fact the model was just plain LARGE (massive resolution that had to be zoomed out to fit right) which was causing some performance issues when all combined.