Yes, I have a similar problem with an older G-11 (single bolt). Funny that Gary!
There is a bit of flex when load is added or removed, and this is what I guess causes the slip.
Not too much you can really do, except keep the weight as balanced as possible. Mine doesn't slip too much, but I'm hoping once it's permanetly installed the problem will go away completely.
What camera are you using? You might be better of taking a series of shorter exposures and correcting for any field rotatation during post processing.
Cheers