I just notice when the ram goes below 10%, it reboots itself. AutoKiller app helps the phone to react fast and prevent ram to go too low. I preset the setting on this app to ultimate performance. so far it didn't reboot after hard used. It has been a day without reboot, but I will keep you guy update to see if this really fix reboot issue. I bought the full version of it.
here is the link: https://market.android.com/details?id=com.rs.autokiller&hl=en
here is the link: https://market.android.com/details?id=com.rs.autokiller&hl=en
I personally have not had that problem, and I think a solution may be near 
