Database Server Hanging up the Network

Hello
My Network has 4 w2k servers.1DC,1application , 1database server,1backupserver.
My nodes are obtaining IP address from DSL modem.
After using 3-4hours the whole network is down.
when i checked by switching off and on the switches and DSL modem,still the same problem persist,so, i checked by switching on and off the servers one by one. ……. i found the problem in the Database server. Every after 3-4hours the whole network is HangedUp. when i restart the database server the whole Network is working perfectly….
so, what could be the problem.Can anybody suggest what to check in the database server.

Nadeem.