Are you using any mods? Do you use that realistic needs setting?
I have a 144Hz monitor. The frame rate exceeds 60 (62-63) even when Vsync is on. Even after installing FPS Limiter for New Vegas I haven't been able to cap it to 60.
I've found the cause: it was performance enhancement from Buffout wearing out.
What the hell mod are you using where Buffout kills you when it debuffs? Unless you're not keeping tabs on your HP, the Buffout chem should only drop your HP back to its former level and so on.
I can now confirm that the cause of death was Med-X wearing of. I reseted my health using console.