Why did you choose such a light tip? Getting weight to the front of the arrow is important on a hunting arrow. The more weight up front, the weaker the spine will act. With your draw length, to get a reasonable weight up front, that arrow will not work - especially at 70 lbs.

That said, the other guys are right, there are many other things that could be causing the fishtailing. Even so, shooting weak spined, broadhead tipped arrows, will frustrate the heck out of you.