Term or expression which best describes a problem that goes away when an expert attempts to diagnose it?
Solution 1:
One term for that which is specific to computing is a Heisenbug:
A heisenbug is a software bug that seems to disappear or alter its behavior when one attempts to study it. The term is a pun on the name of Werner Heisenberg, the physicist who first asserted the observer effect of quantum mechanics, which states that the act of observing a system inevitably alters its state.
(Wikipedia)