You know, having basically gone through the show a few times, the more I think that the whole On/Off thing for Ross and Rachel wasn't really that On/Off at all.

It's off for most of the first series, then its on later in the series, and in the next episode its off, then in a later series it's on for one episode, then off again, a few more series the same thing happens - then it's pretty much off for the rest of the show, until the final episode then its on.