Updated my 10.5.8 to 10.6.8 now the applications freeze every couple of hours.

So, I updated my white macbook about a week ago from 10.5.6 to 10.6.8. At first everything worked fine. Now, the applications freeze every 1-2 hours. I was watching Netflix, when I noticed that while I could move the cursor on the screen, I wasn't able to activate anything in the browser by clicking it. The video would continue but I could neither pause it not refresh the page. I can go from tab to tab, I can scroll, but I can't change anything on the page. At first I thought it could be firefox...except after quiting firefox (it sometimes allows me to do this) I tried to open a text document to type. I can open it, but again I can't do anything within it. I tried to restart the computer, but sometimes the apple menu won't respond. Then I have to either hold down the power button or restart from system preference - startup disk. I thought it might be firefox, so I tried fiddling around with that, disabled some of the extensions, downloaded an older version etc. Same problem.

Anybody have any idea what might be causing this? And, more importantly, how to fix it?

Thanks!