I have a similar problem with a similar setup. I have been using a laptop barely up to the task of viewing SL until today when I dug up my over 10 year old desktop from the mothballs. It is a lot more powerful than my laptop and retired only because it's big and my "new" place had no good place for it. But I noticed a similar problem. While my laptop took up to 6 minutes to load a relatively simple scene with lots of avatars, the desktop handled the scene in a split second (nearly no lag at all) but things still stopped moving and I got logged out from SL 3 times in close sequences on a sim I'