It could be anything - even an electrical issue. I once had problems with USB devices interfering with the firewire port on an iMac causing lost data on a hardware raid. Try removing everything that is not absolutely necessary from the computer’s ports, no daisy-chaining Firewire devices etc. Record to an internal drive.
I am not an electrical engineer but there seem to be all kinds of electrical interferences which supposedly can sometimes even be solved by simply rotating the power plug by 180 degrees in the socket.