Hi mountain cur,
The Burris Rings that Buzzard Bait mentioned are the best permanent fix for the problem your having. These rings are sold in two different offset amounts depending upon how much movement you need.
Also, Weaver is now also making a set of rings that have horizontal adjustability. They are Weaver Top Mount Windage Adjustable Rings and are available in Low, Medium and High. They're good and I use them on one of my rifles, but if I were doing my crossbow, I'd be using the Burris Signature Zee Rings. They are exceptional at correcting the type of problem that you're having.
Many shooters that Buzzard Bait (Russ) and I speak to also lose vertical adjustment at 20 yards after installing an HHA Optimizer Speed Dial. The same rings allow them to re-center their elevation adjustment knobs and use the rings to align their scope properly.
As Russ stated, once you got it, you got for good, so be patient.
Regards,
Jon Henry