Still sounds like the H/G to me.It is not always the case that you will have oil in water or vice versa as it depends on which part of the H/G has failed.Yours could have failed across the combustion chamber to a water jacket causing the water to be pressurised,hence the bubbles in the header tank and rock hard pipes.It is also possible that you have a cracked head, i hope not though.
If you look at my thread in the Technical section, i've just done my headgasket as it was leaking from a water jacket to the outside edge.I had no oil in the water etc.Al.