Intermittent

Intermittance* is the worst thing.

Trying to fix an intermittent fault - which never shows up when you look at it.
Intermittent showers - lots of these today. Hard to plan ahead.
And, according to some comedian on Radio 4 a while ago, the worst possible superpower is the ability to fly. Intermittently.

Emergency blip today. Stop car. Get out. Take picture. Dunnit.

*It IS a word. It IS.