I have a pre ban Colt lower that I have been using OLY Ultramatch uppers on for about the last ten years. I use it for prarie dogs, and shooting local rifle matchs. With the right loads, it always produced sub 1/2" groups at 100 yards.
Two years ago, I wore out the last barrel and have been having problems getting a good barrel installed on my old upper.
Out of frustration, I bought a "Gator" from Clark custom, which is guarenteed to shoot 1/2" at 100 yards. The barrel is made by DPMS, with a 1-9" twist. I have tried every load I could find, and it really trys to shoot a good group, then get fliers, which opens the groups up to about 1" at 100 yards. I noticed, in the last week, that SOME times, after I swing the upper down(for cleaning, or whatever)and reassemble it, it will shoot to a different point of aim, as much as 6" at 100 yards!!!
I know there is SOME slop in the upper/lower fitup, but, there always has been with all the other uppers I've had too, and it never USED to make a difference if the upper was cammed to the front, rear, or just left centered, using the offset pin in the front.

Also, with the 68 grain, and 77 grain match bullets, I've found they will consistantly shoot 3/4" groups at 200 yards, but only 1" groups at 100 yards. I am thinking that with the 1-9" twist, the bullet doesn't stabilize until past 100 yards.

Any ideas???

It's a long shot but...

It's possible the barrel nut has come loose.  Remove the handguards and gas tube and re-torq the barrel nut.

These group sizes suggest a flat top with a scope?

A buddy of mine has wrenchs for AR 15s. I'll ask him to check it for me.

Yes it is a flatop, with a 6.5-20 Leupold VXIII

I appreciate the help, thanks!!