I haven't had consistent frame drops, but I always get HORRIBLE framerate crashes during the 2nd boss of Brayflox hard mode.
Very strange.

I usually have a constant 40+, but after about 5 mins fighting him, I get drops to 2-5 fps that last 10+ seconds at a time.
It only happens during that fight. I get worse frame drops in that fight than I did when I fought Odin.