I'm really sorry about this happening again, but it looks like this time we may have narrowed down the problem!
It seems like there's one mailing list that always causes problems on our discussion list server when something is posted to it. It's strange, the list isn't particularly large or anything.. so it seems like there must be some sort of corrupted data in it's config or a subscriber address or something. We're contacting the owner of the list about the problem and going to work out what we can do with them to get it corrected.
Our apologies again for how long this has been going on! It's really been a matter of not being able to figure out why there was a problem in the first place. Hopefully we'll be able to fix it once and for all now.