Freezing after upgrading from 10.6.8 to 10.9.5

I have a newer Crucial SSD that has never failed. I have maxed the RAM to 8gb. Computer ran even faster than ever on Snow Leopard 10.6.8. However, I got a new iPhone in April and was forced to upgrade iTunes in order to use the phone, music, take photos off, etc. Well, the newer iTunes doesn't work with 10.6.8, so I went to Mavericks. The Simply Mac store did the install. Everything was fine for about 2 weeks before I started getting random freezes. Within a day or 2, my computer became completely unusable. I keep having to reboot. I literally type 5 letters in either a software program or a browser, and it just freezes - the arrow disappears and I have to reboot. I have restored 10.6.8 from a backup, but now it freezes on the old OS too!! Please help. Hardware test at Simply Mac (who installed Mavericks) came up fine.