Blister
Ringer,
If you've got a blister/callous then it sure sounds like something doesn't fit right, thus it is "moving around". Birdie mentioned grip size. If you use standard size grips now, have you ever considered mid-size? I contend that many players use grips that are too small just because the industry has declared them to be "standard" and every set of clubs off the rack comes with that size. You're a pro, so you should know that, but I see good players all the time that use grips that are too small. These guys should know better too, but they don't.
I don't know if you can have too much extensor action or not. Seems like it could be "over-cooked" like anything else in the golf swing. Maybe you should try and back off it some and see if that helps (without losing any structure).
You also may want to check with a doc about that callous and getting it reduced in size. I don't know how large it is, but it may be making things worse.
Bottom line... you obviously have some slip-sliding around going on, and you've determined it's from extensor action. The options to fix it would seem to be sort of limited.
Robbo