This is what I was thinking of suggesting too. Mentioning that different arrows were tried doesn't mean different spined arrows. I have little doubt the 400 spine is too light for the setup. There's an easy way to see though. Reduce the draw weight to at least 65#. About 62# would be even better. Then see if the thing will tune. But remember, you have to attempt the same presision at a reduced weight as you are at 70#. And don't rule out tight nocks on the string. It could be several things causing the problem.