bro i have the exact same problem sometime back also. it's even up to the point that i suspected i have ED. Mine was really psychological as the first time it happened, i fret out.. and the next time it happened again and the next.. for me, when it happened the first time.. it was due to being physically worn out and tired.. but the subsequent vicious cycle was in the mind.. the more i think about keeping it up.. the further it goes down.
i won't say it will work for everyone but what i did was pretty much the opposite by expecting it to happen and knowing that i will be able to bring it back up in a while.. but of course CO was considerably accommodating cos when it goes down. we have other fun instead. then slowly the problem goes off by itself. now occasionally it does happen again but i guess.. used to it until it doesn't matter.