Not sure if my situation was exactly similar to yours , but here is a possible solution I wrote on Amazon awhile back:
I had my RAM (x grip) mount on my motorcycle handle bars on a cross country moto trip. First it performed flawlessly for my iPhone 4. Many reviewers here are noticing that when your phone is locked into the mount, it will rotate or slide into an unwanted position. A partial solution to this that I found is to open up the mount ( by unscrewing the 4 little screws) , and removing some of the grease inside. I just wiped most of it off. This essentially increased the friction so that it was more difficult for the RAM to rotate . My iPhone never swiveled or rotated after 'degreasing' it.