I'm new to traditional archery.  I picked up my first trad bow 3 months ago and haven't put it down except to eat and work.  In fact, I've been following on here for a few weeks and just ordered a Tall Tines bow at some recommendations from fellow forum members.  
The question I have is on arrow flight I'm getting.  My draw is 28.5" on a 55# Bear Kodiak Magnum.  I'm shooting 5575 Gold Tip traditionals with 100 grain brass inserts and 140 grain tip.  When I bare shaft tune, I'm getting everything to hit to the same spot both fletched and bare shafts at 15-20 yards, but my arrow has a lot of vertical flex in flight that I can see.  Do you think it's due to my form or possibly from the short length of the bow I'm shooting?

The most likely cause is that your nocking point is off. You could try moving it up and or down and see if it fixes the problem. If not then it could be your form. You could be applying presure on top of the arrow if you shoot split finger. One way to help this is by having two nocking points one above and one below the nock.

Hope this helps, Schafer