Mutliple NIC in Windows 2000

I have a Windows2000 Server running with two NICs, DLink 528 & DLink 530. DLink 528 is connected to my ISP, the DLink 530 is connected to my internal network. My problem is Windows2000 will switch from one NIC to the other while using the internet. Meaning it will use the DLink 528 for the internet, and the next minute use the DLink 530. What would be causing this? What would I do to fix this?