Problem with WinXP ICS

I have the ff setup:

1 multihomed WinXP PC connected to the internet via Cable Modem, and to the lan via Switch.
Used the network setup wizard to configure Internet Connection Sharing.

When I ping my default gateway (public interface), I could not get a response (request timed out). When I ping a pc in my LAN (private interface), i get a response ( < 1 ms). When I disable my private interface (the connection to my private lan), I am able to ping my internet gateway. As soon as I enable my private interface, the ping status to my public gateway foes from 15 ms to 1000 ms to 2000 ms to request timed out. Before it was working properly, now, I cannot make it work. I have reset my winsock (using winsockfix, netsh int ip reset x.log). Netdiag returns error when trying to ping default gateway. Any ideas?