Your trans was obviously having issues before the filter change, but you induced another variable when you used the synthetic blend. Granted it'* a BLEND, and you only replaced 1/3 of your fluid, but it'* not a good thing.
As the clutches in our transmissions wear, increased lubricity of synthetic fluid can add to the problem. My suggestion is this:
Get a professional flush with non-synthetic fluid, then let'* see how she does and get back to the root problem. 67k is way low for a 4T60 to fail. 100k or more for a SC version, around twice that for a NA. However, your car obviously sat around alot, which isn't a good thing for it.