Thread revival time: Overclocking is definitely not the culprit here (unless your N900 is used and was heavily overclocked before). Prolonged overclocking CAN be the culprit of spontaneous CPU damage if the overclocking is taxing enough on the CPU's electronics to damage it, BUT, I for one, have never overclocked the N900 and I have a nice history with spontaneous reboots. Well, nice in that spontaneous reboots just love me, and my N900 was and still is plagued with them since a LOOOOONG time ago.

I find R&D mode turned on, with watchdogs and lifeguard reset flags turned on to disable those 'features', helps tremendously with spontaneous reboots (if you're willing to occasionally have something crash in the background, that would normally cause a spontaneous reboot by something killing something else, and wait with bated breath to see if it'll restart on its own, or if you'll have to manually shut down the device.) But, for example, just yesterday morning my N900 had either hildon-desktop or x.org or something crash, and not everything restarted. (I suspect it was x.org that crashed because for a moment I saw the kernel's framebuffer console - which normally gets covered - assuming it's enabled in the first place - when x.org starts during boot.)

Some of the stuff restarted, and I got my current window re-displayed. But keyboard input crashed and didn't come back, as did whatever part of hildon lets you switch windows. ANYWAY, the virtual keyboard remained working and I had the chance to finish a PM I was writing on here, before shutting the device off.

So R&D mode can alleviate some of the problems (if you edit /sbin/preinit to disable the keyboard flickering when screen is locked, to keep battery consumption the same) of spontaneous reboots, although, honestly, it depends on the exact nature of the spontaneous reboots.

Worth a try, at any rate.