Computer help: Network connection problems

Network connection problems

Specs
Dell Inspiron 1420 notebook computer
Intel Core2 Duo T5550 1.83 Ghz processor
3 GB RAM
Network Adapter: Broadcom NetLink Fast Ethernet

Here is the problem I am having. Within the last week or so, I have started having problems with my Internet connection dropping a while after I bring up my browser or whatever. I am surfing along and bam, all the sudden the connection is gone. I know that it isn't the router or anything else because the other computers in the home are still connected and able to visit websites. If I reboot the system, then I get the connection back, but after a while, I lose it again.

In this period of time since it began happening, here are the only changes I can think of were done. I installed a card game, which was asking to register, which I didn't do. I installed a number of Vista updates. And AVG anti-virus updated to a new version, and the new version installed a couple of new add-on's to Firefox. And I upgraded to the newest version of Firefox. But this problem also happens with Internet Explorer, and my Google Desktop apps also stop working. Anything using an internet connection stop's working, so I don't think it is a Firefox problem or anything like that.

Thinking that one of these things might have been the problem, I did the following:

Uninstalled the game
Did a system restore to back before this problem started happening
Re-installed the Windows Updates
Disabled the AVG add-on's in Firefox

When I lose the connection, I try repairing it. Sometimes I get a message that if I reset the connection it will fix the problem, and sometimes when I click on the button or whatever, this does fix it, most of the time this doesn't fix it. At other times I get a message that diagnostics has run into a problem that Windows can't fix, and I should restart the system to see if that fixes it. When I restart the system, it does fix the problem - I don't do a restart, I shut it down and then start up again.