My experience is that optical works, or it doesn't. If it doesn't work you don't get anything. I've had that happen before, and I've had dropouts because of it, but I've never had static from a bad optical cable, just "skips" where some music is missing. I'm not saying it's impossible, but that's my opinion.
I have definitely had problems with a humid mic... and the problem WILL come and go when you least expect it. That is what I think your problem is.