You're experiencing an issue that a lot of people run into when they use connection sharing. The 'shared' machine doesn't go out looking for the computers it had been connected with when it wakes up or restarts, and those computers don't know that their connection source has been interrupted, so they don't ask for the connection to be restored.
This is one of those issues where I think the best resolution is to buy an inexpensive cable/DSL router. Only the router needs to be on for either (any) of your computers to connect to the Internet. These products facilitate file sharing very smoothly, and they're easy to set up. You are looking at anywhere from $30-$70 for a reasonably priced model.
If you want to start looking at wireless, you can get a wireless router, which is a combination of a cable/DSL router and wireless access point-sort of like an AirPort Base Station for a lot less money and with more ethernet ports. I saw a deal with BestBuy (I think, maybe it was OfficeMax) yesterday that made the Netgear MR-814 wireless router less than $30 after rebates!