Can't help ypu yet, but I have the same problem. Pipe and O ring fairly new, and don't seem to be the source. Detailed inspection all round here has shown absolutely no evidence. Yet the whole system held 15 psi the day I took it to the garage, while the day before you could see the rad level going down while coolant flowed from the rear of the engine over the gearbox and to the floor. This cycle of leaking badly and then mending itself has now occurred three time sover, and cost a lot of wasted cash.
So I can only offer sympathy at the moment.
I have wondered of there is some heater valve near there, but we can't find one. What sort of major leak will sometimes suddenly mend itself?? All manifold gaskets looked AOK when removed.
Howard, October 2009
