2 NICs and Default Gateway Problem

HI:
I have a problem with my w2k server, I work with 2 Network Cards from different LANS
For example:
NIC 1 IP 10.0.2.8
Gateway 10.0.2.12
NIC 2 IP 10.0.6.8
Gateway 10.0.6.12
When I use the commands ROUTE PRINT I check the default gateway is from the NIC1 (10.0.2.12), but suddenly (3-6 times at day) it changes to gateway of the NIC2 (10.0.6.12)
I want to know how can I kept the default gateway of NIC1, or which is the reason of the change.
Best Regars