Mother board replaced losing internet connect

Hello,
My motherboard had to be replaced last week. Prof charged me $600 to do it... but now my computer will not ping when I am on the net. And while using the internet I lose connection, can't reconnect, and have to restart the computer to get on the net. It can happen ten times a day. Using a mpc pentium D xp system. Comcast has replaced the modem, I replaced the switcher/router. The original reason for the mb replacement was the computer would not start while plugged into the internet. We had to unplug it, and then start, then replug.. that worked for a few days. But then not at all. computer company said the whole mother board had to go. $250 in travel costs, and $350 for MB - I could have bought a new computer. Help!!!