Comment # 1 on bug 1093604 from Takashi Iwai

So it behaves as 48Hz even though xrandr shows 60Hz?  What if you switch the
rate back and forth between them?  Does 60Hz rate work after that, or it's only
when you choose 59.93Hz?

Also, please check without Bumblebee, i.e. to identify whether it happens only
with i915 driver.