I think you'll find it's a spine issue. Your bow is set up for .340 spine arrows. You need a bow set up for .300 spine arrows. As long as both are the same length. This may be achieved by increasing the tip weight but if you're already at 700, more may not be a good option. Or you can tune the bow for a .300 arrow.
How would he "tune" for a stiffer arrow? (Sincere question). Generally, a bow setup for a lighter, more flexible arrow (.340) can shoot a heavier, stiffer arrow (.300) with no problems except a predictable loss of speed. The two main things that affect dynamic spine are head weight and draw weight. More head weight or draw weight will increase dynamic spine, making the arrow bend more in flight and vice-versa. In this scenario the only way to "tune" the bow that would change dynamic spine of the 700grain arrows would be to increase draw weight - something that I think most would agree you don't do if your arrows are flying tail-happy. OP, like I mentioned before, I think you need to decrease dynamic spine (not increase anything) and see what happens. Decrease head weight or draw weight and see. Trial and error almost always works as long as you only make one change at a time.
