Does it reboot if you boot on battery only (charger unplugged)? There's a bug with aging batteries where your unit will spontaneously reboot. Workaround is to get an after market charger, which charges at a slower rate and avoids the bug.

If it still reboots, it really does sound like time for a reflash. You can find those instructions here:

http://talk.maemo.org/showthread.php?t=70096

Hope things go well for you!
Andy