Cluster server with 3 Nics

Hi,

I have a W2K Cluster server with 3 NIC's.

Nic 1 attaches to the Company Network
Nic 2 attaches to an internal switch that we run backups over (no connection to the company net)
Nic 3 attaches to the th other half of the cluster via a cross over cable. ( Hartbeat )

My problem is that one of the servers becomes unreachable on a regular basis. When I run "Route Print" from the command line, it shows the HartBeat address as it's default gateway. If I run Route -f it clears this up and shows Nic one's connection as the default gateway.

Question 1, why does it do this?
Question 2, how do I make it stop?

Mike