Are you using stock software or custom? And if custom, are there any kernel tricks such as overclocking or undervolting going on?
It might be a hardware rather than a software problem though. I'd have thought that a system update would at least change the problem if it were related to system software, and it's unlikely that an app can actually crash the system (though you could try a factory reset to be sure - back up first, of course). Does the crash occur under any particular condition, e.g. when the phone is sleeping or conversely when it is being used heavily?