I remember being a real pain and asking for a 30 fps option a while ago (or, more precisely, it became a 1/2 vsync render option when I understood how c2 runs in the browser). I don't think c2 is mech'd to be able to be able to successfully vary the dt and diverge from what the browser engine is doing. I might be wrong, and it would not be the first time, but but the last attempt to provide an option in c2 to run at 1/2 vsync was a proper failure (after much nagging by me and few others, I recall...).
I would dearly love for us to have a fraction-of-vsync option, where game logic can take more time to do required maths before a draw call, but I don't think it is possible within c2.
I see similar weird behavior on my embryonic game when I test it on an old laptop - it's almost as if the event system breaks down under relatively high cpu load.