For one thing, I wouldn't expect the same arrow to shoot the same on two different bows with a 15 pound draw weight difference between them. Porpoising is almost always caused by the nockset being in the wrong spot, not spine or brace height. The only other thing I've seen cause it is shooting plastic vanes off the shelf instead of feathers. You ain't doin' that, are ya?

You might try some 2117's and see if they'll help.