Sorry for the delay in responding to the question about a term for the situation "You call the repairman to fix a balky garage door, but when he gets there, it inexplicably works." My mother was science-fiction writer Betsy Curtis (no, you never heard of her), and she called it "The Steiger Effect." The premise of the story by that name was that there are people who have a sort of "repairing field" [said Steiger Effect] around them -- which is why, for example, your car runs fine when you have it at the service station but renews the grinding noise after you've driven away. The story appeared in the October 1968 issue of Analog and was nominated for a Hugo Award (losing to Harlan Ellison's "The Beast That Shouted Love at the Heart of the World." (Footnote: Our local service station was run by Mr. Steiger.)
Maggie, I REALLY like this term. Sounds every bit as real and plausible as "the Stendhal syndrome." Thanks for introducing this one into the discussion.