There is a Bugzilla report on the reboot issues. Nokia feels this is actually a number of different issues. Mentioned are: power management, xorg, WLAN, SGX driver, and memory corruption in the kernel (among others). If you haven't already, I'd suggest reading through the posts. There are various bits of information that Nokia has requested that may help them solve the problems.

https://bugs.maemo.org/show_bug.cgi?id=6334