I wonder if the problem isn't something else. Just some thoughts:
Since the Pro Purple is a dry lanes ball, in general, for most people, for today's conditions,
and
since the Gravity SHift is a medium oil (to almost medium-heavy oil) ball, there's a gap between those two ball big enough for at least one ball and possibly even two.
Based on how you say both balls are reacting, I think you're bowling on that great in-between. While the Pro Purple seems to be in its range right now for you, your ball reaction is skid/snap. While its basic designed reaction may not be totally arc-like, but being a solid with a very round core, it certainly is not skid/snap.
2000 or 4000 grit Abralon re-surfacing should help, but then I think you're losing its potential use as a dry lanes ball. If it's your only ball, by all means modify the cover to make it work.
Good luck.
--------------------
"None are so blind as those who will not see."