Well, after a few days of trying all the things suggested by the responses here, I reckon I (you) have found the solution!
It appears that the regulator is stuffed and I was WAY overcarbonating the beer. I degassed the problem kegs completely and left them overnight, then attached a new reg and they pour fine - and carbonation is just about right too.
Since then I've carbonated three new beers with a new reg and they are fine. I checked with my brother in law about his problem and, while his reg is a bit dodgy, he was gassing up at 40 psi (he didn't have gas in the lines just really frothy beer when he poured it).
While I still don't really understand why bubbles come out when everything is under pressure, and now I don't really care, I am extremely happy as the whole problem was really getting me down.
So thanks heaps to everyone who posted with ideas and suggestions - I should've posted the questions ages ago.
Ben :icon_cheers:
