A fluttering speedo is signs of a bad VSS. I am not a fan of throwing parts at a problem. I prefer to use a proper scan tool and fix it once. They run about $20-$30 and take 5 min to change so if it isn't the problem ( which I believe it is) you aren't out a lot. Could also be something with the tru speed. I believe I would disconnest it first and see if your problem goes away. I never have liked those gizmos I prefer to simply change the gear to the right size.
Let me know what this does
NCH