vpn connects slowly

Hi,
I use a gateway-to-gateway vpn to connect the LAN's of my 2 offices. Both
GW's have 2k3-server.
Problem:
To establish the connection it takes long time. the demand dial interface
gets unreachable and if I try to connect it manually it fails and then it
needs long time to get connected.
Each time this happens, the event id 20111 appears in event log:

Event Source: RemoteAccess
Event Category: None
Event ID: 20111
Description:
A Demand Dial connection to the remote interface vpn_staff on port VPN3-4
was successfully initiated but failed to complete successfully because of
the following error: The remote computer did not respond.

Could anybody help me localizing this problem?
thx in advance

Karel