Odd Projector Behaviour

[Julian1]

I have a desktop PC wired to the network via an IP telephone. The PC is also HDMI'd to a projector via a Smart board. The teacher usually uses the projector in duplicate mode. The odd behaviour occurs when the teacher sends something to the network printer (that is in the same room), it causes the projected image to drift halfway to the left. The "lost" half of the image wraps around and appears on the right. It stays like that until the projector is rebooted. Any ideas what's causing this?