Slow network link to office block

I am running a network in a large factory. We have 5 office blocks spread though out the factory. About three weeks ago, one of the blocks just became slow. Files are getting corrupted, daily, and people are starting to scream for my blood. I have changed the fibber tail from the splice box to the hub, tested the main fibber, and changed the hub out but still, it is corrupting data and running backwards. Please, please help.