If you did an Upgrade and Install, thats probably the reason why. About 50% of the time when you do an upgrade and install, it doesn't make your Mac very stable. I would back everything up (Which you should have done before the upgrade) and do an erase and install. I know you will lose your programs and other stuff, but its the best way to make absolutely sure that your system is working reliably. Upgrade and installs are a terrible option, even on the Windows side. Some people can get it to work and work reliably, but most can't.