In chrome, if you move to a different tab than you game then move back again, the fps doubles and seems to be trying to cap it at 120 for me instead.
Doesn't appear to be doing this on Firefox or IE9.
I've attached a capx, but it should be easily to replicate anyway. (Sorry if the download link is slow, having issues with my server!).
I'm running Windows 7 64-Bit
and my Chrome version is 17.0.963.83 m