It could be that your arrows are underspined. Short of buying new arrows I'd try dropping the poundage of the bow to around 65# and see if that helps. It's quick and free. If arrow flight gets worse then the arrows are too stiff, but I doubt that is the case. There is no reason to shoot the bow at 70# if adjusting the weight down a few pounds will solve the problem.
Don't take too much stock in the fact that field points do well out to 50 yards. They don't have any blades steering the front of the arrow so they can be badly mismatched and still group well at any distance.
If you haven;t already done so then spin-check your arrows with the broadhead ferrules in. They must be straight with the shaft and no wobble.