my 2 cents
first thing comes in my mind is loose contacts as sometimes some awkward position or distant shoot site so that cable may have been stretched.
aging monitor as filckring and ghosting does happen with older monitors even lcds
if the problem is with gfx card then it has to be some loose solder some where because if the fan and temperature is the culprit then i do not think for a blink of second it goes out and comes back.
last a thought
if there is leakage of current somewhere in their wiring and the monitor is suppied separately from main board then it may be a voltage drag problem of xray machine (x-ray machine is an electricity hog) so ask them
whether the problem arises just after shooting an x-ray
whether they have any ups or not if not then the monitor must be supplied with separate plug.
whether the power cord of monitor is ok and not cracked
why i am talking this nonsense because i know the doctors using computer and they do not even know the basics of computing even if they studied the fundamentals of science at hsc levels
hope this helps