I unlocked the 4th core on my Phenom II X3 720 BE but it's unstable...

Yes - but sometimes the reason is because demand for the 720 chip exceeds the supply of 1 bad core chips they have on hand so they take perfectly good chips and disable the core - THis is why there are some chips that are perfectly stable with the fourth core unlocked ( I happen to have gotten one that has been running for about a month 24\7 with the 4th core unlocked and a small OC to 3.0 GHz. so they are out there !) - Seeing that it was bought in the begining of DEC. just before the start of the christmas shopping season they probably had to make some 720's with perfectly good fourth cores to fill the demand they would be getting during the holiday and I was lucky enough to get one.

That being said though you are correct that in this case it would seem there actually is some defect in the chip so best thing to do would be turn the 4th core back off and use the chip as a X3 like he bought.

When not enough volts are being provided to the CPU it will reboot at any time (Especially during heavy to max loads) with either a black screen or verticle lines right before rebooting. While posting you see this messege.

"hypertransport sync flood error has occured. Press F1 to resume"

So there are a number of Scenarios that may cause this BUT they ALL lead back to the main reason. Not enough power to the CPU at the time that the heavy load produces failure.

1. To high of an OC without the ample amount of CPU volts.
2. Plain, not enough CPU Volts at any speed.
3. Possible Bad power supply.
4. Motherboard with very high amount of VDroop which ultimately leads back to "Not Enough CPU Volts".

With the most recent Bios and the correct amount of CPU voltage you should not see that error message again...

So you arent going to try to update the BOIS to see if it fixes it? Cus i unlocked my 720 BE and man is it ever nice.