With N900 two years back me and my colleague troubleshooted one that was unstable for a week; in the end it just turned out that we found out about the powersaving calibration error that could've been curable with later firmware versions, but it'd have taken at least two months (next release) to prove that it'd have helped in the first place.
Powersaving features causes lots of problems in servers, so it is no wonder that a chip can be badly calibrated or plain faulty in phones too.

Waiting for a new firmware would have been two months for him with one or two reboots per day, while mine was solid. And there was no any kind of guarantee that it'd have helped. It was over much faster by letting Nokia care handle it, even if it took some many days to get it back, so that's why I think you shouldn't torture yourself with trying to troubleshoot it too far.