This sounds exactly like the problem I've been having. As you said, it stops on its own eventually. Sometimes after an hour, once as long as 10 hours. I have booted in safe mode, and it happened there as well, indicating to me it's not an app I've installed, but something with the OS. I've also removed the SD card and it happened without it, as well as with a brand new SD card. I have NOT tried a factory reset, which I know it probably the most obvious thing to try, but I've been procrastinating. Plus, I read on another forum that people with similar problems have tried that, and the problem reappeared.
I've noticed that it mostly (maybe always) happens after my phone runs out of battery and dies. Once I turn it back on (either while charging or fully charged again), it goes into bootloop. I've been debating having the phone replaced, but so far have just been trying to avoid allowing the battery to die (not always successfully). It also didn't start happening until about a week after the 2.3.6 update (soak test).
Let me know if you learn anything more about this, and I'll do the same!