This seems to be a common enough occurrence that it would merit its own phrase. I imagine it might be some sort of subset of Murphy's Law. But it's specific enough that Murphy's Law doesn't quite address it.
Do you know when you are banging your head against a wall for hours trying to fix something? And then when you finally give up and try to get someone else to come fix it (or even just look at it to see that it's broken), it's working perfectly as if nothing had ever happened? And at this point, you're just trying to convince the other person that it was broken in the first place.
It'd at least take some of the frustration out of the experience if you could both just laugh about
<insert the phrase for it here>. But as it is now, I basically have to go through the whole paragraph above to say it, and at that point the joke is kind of lost.