This is a post for future reference and may help solve some notebook crashing problems.
For several months my Dell Inspirion 6400 crashed continuously and often. Many times I could not stay in SL more than a few minutes. I would log in with good bandwidth and over a period of anywhere from a minute to several minutes I would watch bandwidth fall until it finally crashed. SL bandwidth fell but not the bandwidth coming from my ISP. This would happen even if no SL lag and of course was much worse when SL lagged heavily.
I use a NetGear wireless router but had the the notebook computer connected to the NetGear router directly by cable. The wireless router also made a connection to the Intel wireless card. I understood that when the cable was plugged in the computer used only the cable signal. Wrong. When just by chance I went into the computer's wireless card settings and turned off the radio it completely solved the crashing problem. No more gradually losing bandwidth. Been enjoying SL ever since and able to stay in for long periods. Evidently the computer gets both the signal from the cable and the wireless signal and that causes a problem with SL.
I hope that this helps someone suffering the same frustrations that I had.