I'm not aware of any limitations in the eval version of W2K8. If W2K8 is true-to-form with other Microsoft Windows Server evals, it should work exactly as the full version until the trial period expires, and then will periodically STOP (panic, blue-screen, etc) but continue to work otherwise. (Lots of fun when a classroom full of computers gets their dates set ahead by another class and you find that all your student VMs are eval-period-expired and blue-screening like crazy for the last 10 weeks of a 16 week class... >smile<).
Does the machine still respond to PINGs? Does the console appear normal? Does the mouse pointer move on the console? (These sound like silly questions, but I'm trying to determine if the machine is still processing the kernel scheduler event loop, or if it has locked into some kind of infinite loop.)
It seems unlikely that a user-land process could cause what you're seeing. Even so, I'd start logging performance counters, I suppose, if nothing was showing up in the event logs. I'd grab the "Processor", "Process", "System", "PhysicalDisk", and "Network" objects to try and correlate the "lock up" to IO events.
I'd suspect dodgy hardware or a bad hardware driver as the culprit. Bad RAM would typically manifest sooner than you're describing, but it's a possibility. I'd be sure I was up-to-date on all my hardware driver versions.