I think it's good to know the following from all the users with problems:
- Guruplug Server or Server Plus?
- Is your plug on 110v or 220v+
- Do you have ethernet connected? and if, only ETH0 or 1 or both?
My latest guess is that the rebooting issues are not power related, nor heat related.
I think that it has something to do with the ethernet drivers (in both u-boot and the kernel, or one of the two).
See a patch submission to u-boot back in December 2008, this is about the Marvell 88E1121 (the PHY in the Guruplug Server Plus);http://firstname.lastname@example.org/msg05529.html
I know my plug is stable when there is no ethernet connected to it. The moment I connect ethernet to it, it becomes unreliable. Unfortunately I do not have the required knowledge to "debug" this deeper, but I think I'm on to something.
So those who have issues with their Guruplug, let us know what and how you have stuff connected, and on what voltage you are running your plug. (although I do not think voltage has anything to do with it).