I have a brand new Photon that came out of the box with MR-1 (but the new "padded" back cover to keep the battery in). It would either shut off or go into the death sleep every 10 minutes or so. It does it so often, I have to keep the back of my phone off since I pull the battery so frequently.
I factory reset the phone and installed MR-5. Install went fine and the build is now 4.5.1A-1_SUN-154_5 It was reported that this build fixes this shutdown / sleep problem in addition to other issues. I've only experienced the random shut down / sleep of death problem.
After upgrading, I updated the PRL and did a second factory reset to be safe. I entered my Google credentials and stopped customizing at that point.
It still shuts off just as frequently as it ever did. No change. I can't tell if it is a full shut down or a sleep that it can't wake up from. The phone does get hot so I am guessing it is/was doing something that crashed everything.
If the phone is on the charger, it never dies. As soon as I take it off charger and put it in my pocket, moments later, I can't wake it up and have to remove the battery - again.