What model HP server and what kind of drives? I've got four DL180G5 servers in the datacenter used for CCTV recording and had nothing but trouble with them last year munching drives. Symptoms were similar to what your describing with the logs being clear and the filesystem randomly locking up. Given enough time individual drives would fail. It turns out a number of 750GB and 1TB drives had a faulty firmware on them. Check the models of each drive and get the latest firmware and do it for the controller as well.
The only other thing I can think of is maybe jumbo frames got turned on somewhere. If a client/server is using it and one or the other/something in between doesn't support it then you'll see stuttering on the network links. This wouldn't account for the server physically locking up though - drive firmware would.