Hello Graeme!
I had experience with many old i1Pros, from my experience there were
also some issues with USB power. So I think good USB cable is very
important for every i1Pro.
Also I would suggest to unscrew filter and check it for the dust.
Best regards,
Alexey Gribunin.
Less likely to be USB lead then. I notice that one of my i1Pro Rev A's sometimes
fails the first calibration after being unused for a while. It passes on
subsequent attempts though. My thought is that it is a sign of the lamp
getting old.