my dad's sensation 4g from bell canada has had this issue as well, stock, stock, stock. gingerbread still.
he had HTC send him a new battery, didnt help.
over a week and a half ago, changed the launcher from sense to ADW, and has not had a shutdown since.
he thinks it may also be the beautiful widget he added at the same time, but i tell him no..., he thinks the shutdown issue may have started when he removed one of the stock widgets.. but no... launcher... maybe.
not saying it is a fix, but possible, waiting for official ICS OTA before we void the warranty, in case it is a hardware issue.