Ever since tango my 900 has been randomly crashing too. Its usually just as I turn the screen off. A few seconds later ill go back to see if I missed a txt, or do whatever and my phone is off. Holding down the power button turns it back on. However there have been a couple times that it happened and when i turned it back on, it went in to a rebooting loop where it would start to boot up, nokia logo would come up, then it would shut down again...wash, rinse, repeat for a good minute.
With the problems Ive had with my 900 I will most likely go back to Samsung (who i hate) if I am absolutely jonesing for a WP8 phone, but if I can hold off a few months, and the 920 doesnt seem to have so many problems reported, ill give them a shot again. its only fair since i gave samsung 2 shots (at least their hardware worked, i just hated how they felt).