........and when you report it, you get told "It always does that" or "You have to thump/ twist/ tap/ squeeze/ pull/ push or wait thirty seconds and it comes good" with the engineers unspoken entreaty "Please don't write it on the MR or we are both in trouble".
I know a few aviators who when told that would return the aircraft to the engineers a few days later having 'thumped' it through the instrument panel into pieces so it failed permanently, with the appropriate MR entry stating the instrument had failed for the last time.