Why does the Desire Z get unstable all of a sudden? topic

I have a odd bug that I think has to do more with the Desire Z than a ROM....

It can be 5 days straight stable, no reboots nothing........and one day....all day its just rebooting and looping and rebooting and looping.......and the next day, its stable again for weeks....

Ive tried to clean cache and all but I cant seem to make this odd pattern go away.

I dont install anything during these periods.