My Mac freezes and only mouse moves

Hello, I have problem with my iMac 20" mid 2007. It randomly freezes once a day and if it freeze I can't do anything if I'm listening to some online radio it keep playing but everything else freeze I can't close apps, minimalize, cmd + shift + esc do not work, only I can do is move with mouse cursor and to get it work I have to do hard reset. I don't know what can cause this but I think it might be Safari. Because of since yesterday I've installed Google Chrome and I do not use Safari and it seems it will not freeze(I hope) and It freezing only if I'm browsing the internet with Safari. Also after the upgrade from Snow Leopard to Mountain Lion shutting down takes about 20 - 30 seconds with Snow Leopard it was about 1 - 2 seconds, so should I reinstall and if so to Lion or Snow Leopard. Because iMac 20" mid 2007 was design for OS X 10.5 and I'm using OS that is 3 versions newer.

Thank you so much in advance.

All help will be appreciated.