FastSync dropping smoothness but FPS stays the same

[MIETAS]

Hey,

I've been experimenting with fastsync + purexp + custom 120hz res(settings provided by Chief in other thread) on my XG270 and some weird problem occured:

when i have config mentioned above - there is a random and frequent drop in smoothness while ingame. FPS stays the same at locked value - 120 but game suddenly behaves like its in 60hz or worse.

Any ideas what might be a problem here?

UPDATE: looks like the cause of this was wrong pureXP setting. I mean not technically wrong, but not compatible in this config.
Before it was set to "normal", changing it to "extreme" solved a problem.
Bug? No idea.