My E51 reboots randomly by itself. many other people seem to have the problem .(see http://www.howardforums.com/showthread.php?t=1242623&page=96&pp=15 )
It particularly seems to reboot when using E-GPRS(EDGE)
for internet.EVEN when i am browsing on PC via USB or BlueTooth E51 reboots. I have not been able to browse web for more than 3 hours without the phone rebooting.
Its a silent reboot.NO hands animation ,No nokia startup tone. Only nokia is written on white background and then standby screen appears.Standby Plug ins like "Setup voice mail if hidden " if hidden BUT not disabled Reappear.(A nice indication that your mobile restarted in your pocket)
Many people may not know their Phone rebooted as it is silent.I only realised after a week when i hooked up the phon for browsing.
People have even mentioned rebbots while in a call,thus disconnecting the call