Please be more specific about exactly what your script was doing and what the browser was doing when you got this error. My guess is that you tried to send keystrokes, mouseclicks etc to Netscape before it was ready. Netscape is much more brittle than IE and will crash if you don't use great caution. Most likely calling Sleep() at the right time will be how you fix this. I am assuming that your extensions and default browser are setup correctly.
Chris R Mead
The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.
Check memory usage by the Browser: likely it is crashing when free memory is slowly consumed over 3 hours (I usually crash by 2 hours). This can be done with SYS_GetMemoryInfo calls.
I believe there are a number of threads within this forum that will provide details for restarting the Agent. Any solution chosen should Disconnect the Agent before restarting otherwise Agent crashes and SilkTest crashes/hangs/bad channel's will very likely occur.