I know what you are trying to do now, but I'm still trying to look into the problem myself. I don't think you can do that, if you have two default gateway on 1 adapter, then the obvious solution is to lower one of the metric but this isn't the case for you since you want both to be 30 but still use a particular one. I don't know if the order of the default gateway will matter in this situation.
Here's where I'm reading it from
Read the part about the "Dead Gateway Detection" also. This is the thing that probably cause it to keep switching.
I guess what M$ was saying is that in Windows XP when there are multiple default route in the routing table with the lowest metric, TCP/IP for Windows XP and Windows Server 2003 uses the default route corresponding to the adapter that is the highest in the binding order. You can view this order by going to Control Panel/Network Connection. Select the Advanced menu on the top and select Advanced Setting. The binding order is listed in "Adapters and Bindings" tab. Not sure if this is it, but since a default gateway and default route are two different things use interchangeably