Router VPN Log shows "Sending phase-I notify of type R_U_THERE_ACK" repeatedly

I have Netgear VPN Firewall Router FVS124G.
It has a VPN link to 2 other routers.
For about a week ago now the vpn log shows:

TUE JUN 17 15:25:50 2008  INFO :: Sending phase-I notify of type R_U_THERE_ACK
TUE JUN 17 15:25:51 2008  INFO :: received NOTIFY PAYLOAD of notify type R_U_THERE
TUE JUN 17 15:25:51 2008  INFO :: Sending phase-I notify of type R_U_THERE_ACK
TUE JUN 17 15:25:54 2008  INFO :: received NOTIFY PAYLOAD of notify type R_U_THERE
TUE JUN 17 15:25:54 2008  INFO :: Sending phase-I notify of type R_U_THERE_ACK
TUE JUN 17 15:26:01 2008  INFO :: received NOTIFY PAYLOAD of notify type R_U_THERE
TUE JUN 17 15:26:01 2008  INFO :: Sending phase-I notify of type R_U_THERE_ACK
TUE JUN 17 15:26:02 2008  INFO :: received NOTIFY PAYLOAD of notify type R_U_THERE
TUE JUN 17 15:26:02 2008  INFO :: Sending phase-I notify of type R_U_THERE_ACK
TUE JUN 17 15:26:03 2008  INFO :: received NOTIFY PAYLOAD of notify type R_U_THERE
TUE JUN 17 15:26:03 2008  INFO :: Sending phase-I notify of type R_U_THERE_ACK
TUE JUN 17 15:26:11 2008  INFO :: received NOTIFY PAYLOAD of notify type R_U_THERE
TUE JUN 17 15:26:11 2008  INFO :: Sending phase-I notify of type R_U_THERE_ACK
TUE JUN 17 15:26:12 2008  INFO :: received NOTIFY PAYLOAD of notify type R_U_THERE
TUE JUN 17 15:26:12 2008  INFO :: Sending phase-I notify of type R_U_THERE_ACK

It won't stop and users have been reporting slower connection speeds.

How can I tell what is causing this?

Any help would be appreciated.