how many machines in each office, and how many users on the vpn?
my first thought is that you have a broadcast storm created because you are using a bridge
if you dont need a specific layer2 protocol, you will gain performance and quite possibly fix your problem by changing to a routed setup.
here is how to link 3 lans with a routed setup:
http://www.secure-computing.net/wiki/index.php/OpenVPN/Routing