It does seem like the computer is overheating, hence the random restart, and by booting it up immediately after one of the restarts would definately cause another restart, before it gets to windows.
Also, radiator temps shouldn't have an effect would it? unless of course, it is extremely close to ambient temp, which would indicate that no warm water is actually getting there....
Also, I'd be interested in seeing how you managed the waterflow: is it going to the GPU? and if it is, is it going there before, or after the CPU? I'm not sure if overheating the GPU would cause a restart, and if you're running hot water from the CPU to the GPU, the GPU temp MIGHT be above normal. And vice versa.
Also, not sure about the instructions on that WC202, make sure you didn't overapply thermalpaste, although I'm sure if you overapplied it wouldn't be as bad as this.