I've a Vienna 1 Deluxe, and have that happen now and then. I just move them around, finding little variations seem to help. Also my ergo grips twist some and I found that angling them help. Consider some mountain bike bar ends. http://www.jensonusa.com/store/sub/1094-Bar+Ends.aspx

Good quality gloves help too.

FWIW I had the same issues when I rode motorcycles and solved it just by moving my paws around.