Loosing connection with Cable Modem

I have two NICs on my Windows Adv. 2K. I’m running DNS and DHCP services. The problem is that most of the time when some one in my organization wants to use the internet from his/her workstation, then cable modem starts loosing its connection. The leds on the modem are jumping around. Sometimes I have to reset the modem by turning it off and on and restart the server. Look like incoming is fine but outgoing is the problem for me. What should I check with my DNS. I thinks it causes the trouble. Thanks in advance.