Misfiring especially when damp

Hello, I'm new here and I don't think this issue has been covered exactly. I have a 2004 Nissan Micra (K12) with a long-standing misfiring problem. A well-regarded garage has looked at it (including bringing in an auto electrician) and has drawn a blank after trying all the usual things (plugs, coils, battery, connections). The engine diagnostic light comes on (sometimes) and the code is "Random misfire" - I don't think the cylinders are specified. Sometimes the car will run fine for weeks on end, with or without the warning light on. But in very damp conditions it misfires badly and sometimes, like yesterday, gives up completely though it is OK again when it has dried out a bit. A local Nissan garage says they have seen the problem on 3 cars of similar vintage recently and each time they have had to give up and scrap the car - even a new ECU hasn't helped. We are about to give up but I wondered if anyone had actually managed to solve this problem. Thank you.