When I did my Merchant Navy Radio Officer's ticket, back in the 60's, we were taught that with an intermittent fault, give it a good thump. It would either stop working completely, in which case you could easily fault find it, or it would work, in which case you could get on with something else!
Two blondes checking their indicators. Yes there working, no they're not, yes they are, no they're not, yes they are, no they're not....................................................................