Mac OS X forum: 10.6.3 - "crashing" with SuperDuper backup and download

after transferring successfully all apps, etc. from my MacBook which runs 10.5.6 to a new MacBook Pro running 10.6.3 everything is fine EXCEPT:

1. when downloading large map file from Garmin website to my GPS's MicroSD card running Safari, which took forever, I came back to computer after awhile and there was the login screen - either MacBook Pro did a Restart or a Log Out - this happened repeatedly & I gave up on the download - I never saw it actually happen as I just couldn't watch the computer every second for the hour or so this download went on.

2. Exact same thing happened (either a Restart or Log Out all by itself) when I was backing up to a Firewire external drive using SuperDuper, no other apps open - the first time the backup did not complete according to SuperDuper log after I logged back into MacBook Pro; the second time, the backup was successful according to the SuperDuper Log, but still, when I came back to computer after walking away during the backup, there was the Login screen.

3. Same thing happened when I did a Time Machine backup the first time! The second backup attempt with Time Machine did work as far as I can tell (although the log info, etc. with Time Machine doesn't seem to exist, doesn't say "backup successful" or anything) and also this second time backing ujp with Time Machine DID NOT trigger the restart or log in thing.

I had several times in past done a similar download from Garmin, and many, many times backed up to external Firewire drive using SuperDuper on my older MacBook running 10.5.x and NEVER saw anything like this happen. Again, nothing changed between my MacBook 10.5.x and my MacBook Pro 10.6.3 other than system -- all the apps are the same - this is a brand new machine and I just used Migration Assistant to copy over, etc.

I thought Norton Antivirus might have been the problem, but with all protections turned off, the 10.6.3 MacBook Pro STILL either restarted itself or logged out (I can't tell which - I just know that when I returned to machine I had to log back in, etc.) I also tried turning off screensaver and all Sleep options, including hard drive but this didn't help.