From the information included below, it seems that possibly the bridge cannot handle the amount of traffic that is being sent and received via the bridge. If you are not on the latest firmware for the 340 bridge, you may want to consider upgrading it to the latest.
*Underrun PacketsPackets failed to be sent because the access point was unable to keep up with the Ethernet controller
*Overrun PacketsPackets that were discarded because the access point had a temporary overload of packets to handle.
These definitions were taken from the following URL:
http://www.cisco.com/en/US/products/hw/wireless/ps458/products_configuration_guide_chapter09186a00801212c1.html