I've experienced the exact same issue since the "upgrade" to ICS. I remember when I first got this phone that it was experiencing spontaneous reboots but it eventually was corrected. But now I can't even get the phone to run for 5 minutes without it shutting itself down. Because I use my phone for work in the field it makes things very difficult. I took mine into my local store and was told that the reboots would eventually subside as the new operating system updates everything. But the problem seems to have only gotten worse since then. Hoping that something can be done very soon!