The first thing I would check is the runout of the flywheel. The only thing that usually causes chatter is warped surface or hot spots on the flywheel. You also need to check the surface of the crank flange for runout. This is unlikely though because from what I read from the post it was only slipping with no chatter. That PP you have in the picture looks fine to me because I have run worse than that before with no chatter.
I would try another "known good" flywheel if you can find one. I have also seen this issue when someone let the transmission hang from the bell while searching for the bolts, it had warped the hub on the disc.

Gus