I have the exact same problem, but on a brand new top of the range Mac Pro with all the trimmings.
I can only edit for about 3 minutes before the program monitor starts to freeze up, after which there is nothing that can be done so I have to force quit Premiere and re-open. This is wasting a lot of time and is embarrassing when you try to show a client the project and it keeps locking up. If anyone has a magic solution for this I'd sure like to hear it!