If the system has worked well for about 2 years and now something is happening, sounds like it is related to a change at the customer's site. Did the customer add any electronic equipment (motors, lighting, etc...) to the same circuit the sound system (or parts of the sound system) is on?
There could be an issue with the connections for all the gear. If not already, you (or maybe the customer), should go through and disconnect/reconnect/disconnect/reconnect all of the inputs/outputs of the entire system (including all network connections). Anywhere there is a connection, it should be exercised.
Also, it's assumed the network cabling is all Cat-5e (as a minimum)?