ORIGINAL: buckeyebuckhntr
If he is shooting a 100 grain broadhead currently and was shooting a 100 grain field tip before it isn't a spine issue IMO. A 100 grains is a 100 grains when you compair mechanicals to field tips. I would suspect blades opening on route to target disrupting the arrows flight.
The Spitfire is a very long broadhead with more surface area than a fieldpoint. A borderline spine that shoots small field points great, may very well show this disturbance with the larger profile mech (plus it changes FOC). My 2002 Pat SC shot great groups with field tips, but shot NAP scorpions (similar in profile and length to the spits) all over the place last year w/ my 3-49's. Went with Rocket Steelhead 100s which are teeny and the groups tightened right back up. AfterI purchased another bow for my primary hunter, I shot stiffer spined arrows with the Scorpions out of the PAT for kicks and the groups tightened right up.
However I think this may be a tune issue, or is a very common problem of misaligned inserts...even with mechs, misaligned inserts can cause flight deviation such as this...especially with the larger longer mechs designs.
Make sure the shafts are straight and the inserts are aligned properly.