There are many things at work here.
First, we do not know the base material and that is a big factor, that aside...
Is the root radius of the tool properly sized, it may be too large?
If you are using a "Full Form" insert, try a small root radius non-forming insert.
in your G76 code vary the infeed angle, try a full 30° per side infeed.
Is the tool on center?
Is the thread very long?
This should get you started, let us know how you make out.