Help - Canon EOS Rebel T3i - mirror stays up on shutter release

[vincentllora]

On my canon rebel t3i when pressing shutter button the mirror goes up correctly, but never comes back down. I have to turn the camera off and then it goes back down. I have set the camera back to factory settings and ensured it has the latest firmware. It does this behavior with any lens on or even if the lens is off. It is very clean, nothing 'gummed up.' Manually trying to move the mirror does not 'free it up.' Manual lockup is turned off, but I've toggled it on and off with no change in results. I have a fully charged battery and have tried multiple batteries. I have tried with and without an SD card installed. I have turned auto clean off and on.

The only thing that seems to have temporary success is that I reinstall the same firmware and then it functions correctly, but only for about 6-8 times. Then the mirror stays locked at the top again.