I am having an interesting problem with my service rifle. It is a Compass Lake Upper, and it has taken to the groups shifting up. The group size is still the same, but moves up. I first noticed this a few weeks ago durring a match. When I fired my sighters, they were about 8 minutes high, so I made the neccessary correction. Durring the sitting, I noticed that the group was a little high, but didnt think much about it. After my slow prone, the entire group was high again, great group size, but only two scorable shots on a 200 yrd reduced target. Went through the rifle when I got home and couldn't seem to find anything wrong, so I took it to the range and sighted it back in at 200. Went back three days later and I was back at the top of the target with nothing in the rings.  Also I use a 6 o'clock hold.
Thanks guys, any ideas would be helpful because I am out of them.

Are you using a tight sling?

Yes, in prone and sitting, not in standing. I do have a float tube under my handguard and my front swivel is attached to the tube, not the gas block. It was shooting highest in prone while using a sling, and that is the same position that I used to sight in the rifle.

Is your rear aperture the two-position type?  If so, it can shift under recoil and shift your group up, especially if it bumps your glasses.  You can fix it by gluing it in place or switching to a fixed aperture.  This would be a good opportunity to get John Holliger's modified sights (see White Oak Precision).