I picked up some 100gr field tips to mess around with. You are correct.... The 100gr field tips shothigh compared to the 125gr field tips (and didn't porpois or fishtale). I bareshaft tested by bow too like you and others mentioned too with a 1/2 trashed XX75 that I cut the fletchings off of.

With a 125gr field tip on a bare shaft from 20 yards, it flew straight with no wobble, porpois, fishtale, and hitthe bottom of the target.

With a 100gr field tip on a bare shaft from 20 yards, it scared me... It flew almost like a corkscrew and was porpoising like a dolphin! [&:] It hit the target on the front edge at an weird angle too.

This experiment told me that my old bow is setup and tuned for2315's cut at 30.5" and 125gr tips, and that's just the way it's gonna be... Why the 100gr Stingers shoot straight and dead on accurate is beyond me. I am going to order some 125gr Slick Tricks now and see what they are like compared to the 125gr Thunderheads.