More clues are needed, but is the machine free from parasites? Read http://www.doxdesk.com/parasite/
And how about resource levels? Read http://www.aumha.org/articles.htm
Can you try the Firefox browser? What happens?
I get this pretty often, along with "cause unknown" why? how do I fix? It drops me from the net; and goes back to my start-up screen. I don't have to re-dial, just click on "E", and I'm back on line. It's just agravating.