Slacking Modem

A recent upgrade of motherboard, CPU and Memory has caused the external modem to start acting strange. It will connect and verify the username and password, but any attempt at FTP or HTTP communication will usually time out. Have tried two ISPs (neither recommend using proxies, so we dont; but have tried with no success) as well as a different modem, phone cable, modem drivers, and serial port. After trying a different motherboard i am left with the questions, how can this happen, and how much does the CPU factor into web browsing?