I was setting up one of my long bows for an outdoor NFAA shoot in PHX.  I wanted to use a lighter set up than what I hunt with to make the 80 yard shots a little easier.  I'm shooting a 60" Addcock 58# @ 28", I draw 28". The arrows are 29.5 with 100 gr brass up front with 145gr tips.  So I step out back and let fly,  I'm 4" left of center.  So I say that's weird and try again.  Same thing.  So I go back in the house and screw on 175 gr tips.  Now I am 10" left of center.  To make a long story short I ended up with 100gr tips to hit center.  I had always thought if you went left it meant that you were over spined not under spined.  What ever works, but I'm still puzzled.
MAP

Sometimes you can be so underspined that your shaft can contact the riser causing it to fly left.  Could that be your problem?

Always double check! I do the same thing with BH!

Could be you are still too stiff and you are pushing off the riser.  The stiffer you go the more you push off and go to center or weak side.  The weaker you go you push off less and start to come back to center, but you don't get there quite yet because you are still too stiff.

Hard to say without more info.  Do you have longer shafts, heavier than 175 points to test the idea?

I didn't see wither you are left or right handed?

I am right handed, I see that I didn't say what the spine was, they are 55/75 GT's.  I tried my 250gr tips, if I had held center they would have missed my backstop.  I took this set up [the 100gr tips] to our indoor 3-d shoot last week and shot 192 which is an 8 point average per shot.  I raised my knocking point 1/16 to get them shooting a little lower.  Its just a little strange is all.  I've never had this type of response to adding point weight.
MAP

Hi Michael I shoot Bemon ICS 400 Hunters on a 55# @ 28" Drawing 26 ¾" witch put me at 52# on the scale. Using the three bare shafts and three fletched arrow method out lined here  http://bowmaker.net/tuning.htm
My arrows are 29 ¼" with 125gr tips or broad heads with only the regular inserts and at 15 yards all 6 arrows come in at a 1 ¼" group. I started with 160gr and worked down to 125gr for best flight. My fletched arrows hit left of center while my bare shafts hit left of center. Oh I'm right handed.