Butch
Two things come to mind on your topic. First have you tried lapping the base rings?. Since you are sure that the scope is okay,and have secured bases as suggested on the other posts, I would try this.
Your scope probably has a 1inch barrel, so get a piece of 1inch cold roll and some lapping compound, a couple of passes would tell if the bases are running true to each other.
The second thing that comes to my mind are you shooting different ammo each time, but you probably know about that.
Good Luck and post your findings as I'm sure a lot of us are interested, I know I am.

Thats close enough for the girls I go out with!