'Intermittent' connotes an on-again-off-again pattern with irregular interval, whereas 'sporadic' connotes no pattern whatsoever. It's a slim distinction on paper, I'll grant you.

However, when someone says "[x] is happening intermittently", the implication is that [x] correlates with a background process they can't identify. When they say "[x] is happening sporadically", that implication is absent. The distinction provides evidence as to where the problem lies.