Paul; the reason I'm polling is that I want to try to determine whether it's just me or whether it's a common problem.  And no, I don't know anything you don't; I'm hoping that this process might help find a solution.
 
Jon; I have an 86GB hard drive with 30GBs free.  And yes, my laptop is connected to my network via a wireless connection.
 
Until you suggested clearing the cache, and I went and read the help file, I didn't realize you could clear the cache without clearing the history; and it's very helpful to me to have 30 days of history available.  So, now that I know you can clear the cache while leaving the history intact, I'll try changing the settings to automatically clear the cache upon closing, and see if that helps.

Dave, if you have the option to plug into your router with a cable, instead of wireless, it's certainly worth a try for troubleshooting.

That's a good idea; thanks Adriaan!

if the wired connection works ok, you may want to re-install the wireless controller.  I had a similar problem with the Intel Proset Wireless controller.  Uninstall and reinstall cured it.

Some network cards are also set to go to sleep, as an energy saving option:
 
Control Panel
> Network Connections
> Local Area Connection
> Settings
> [network adapter name] Configure
> Power Management
 
Make sure the option for Allow the computer to turn off this device to save power is not marked.