Argo Blanco

I'm running Second Life from a notebook, with a Nvidia Geforce 9300m gs 512mb. Notebook is an asus with 4 GB RAM, 2.53 Ghz clock. When i log in, the first 20-25 minutes, SL runs smoothly and fast. After this time i experience drop in FPS, from 22 to 0.8, Ping sim goes from 280 to 1600 during this matter. After a pair of minutes it return running normally, for a few minutes and then down again and up and so on until SL is completely useless cause the time span between the normal running and the drop of performances decrease progressively. Anyone do know what this is due to or how to fix the problem please? P.S: Graphic card is up to date, my directX are not of the last version, my O.S. is Vista