I'd bet your bow/rest is slightly off tune left/right. Any issue in tuning will be emphasised with broadheads. Perhaps being slightly underspined isn't helping the situation as DaveC has pointed out. Unless your seeking a higher end FOC, you'd be better off with 100 grain heads. If you have access to a laser tuner, you could check your center shot as well as the simple scale models that go in your limb bolts...if you feel you are where you need to be, slightly bring your rest in about a 1/16" and see where the Muzzy's are grouping compared to field points, another 1/16" may be required but be as close to center shot as you can be.

Have you done anything since this post?