Due to the difficulties I was having with Ubuntu I finally decided to just try reinstalling everything. (Fortunately, I have my home folder on a separate partition so I didn’t need to reconfigure too many things.) The reinstall turned out to be a really good idea: all the major issues I described are gone. It would appear that all the problems were related because the sluggishness I was experiencing when I switched windows is gone too.
I had a little trouble getting the network working again apparently due to the way I had configured things. When I installed Ubuntu 4.10 I was unable to set up the onboard network adaptor from my motherboard and so I had installed a separate adaptor card. Now with 5.04 the onboard adaptor is working just fine. However, this caused a problem with the way my gateway router is set up.
I’ve reserved IP addresses for all my computers based on the hardware address. But, now that my Ubuntu computer has a different adaptor the gateway DHCP server assigned it a different IP address. It took me a few tries, but eventually I got around to updating the settings on my gateway and I was again able to connect to the Internet. Given that I’m still a wee bit fuzzy on the details I may have missed the real problem (obviously I’m a considerable distance from being an expert). But, my computer is working well again so I’m happy. In the future I’ll have to be more careful about trying to change the configuration.