Hi superspud and welcome.
I'm likely to be of no help here, but I like a chat, and if you've been replacing vacuum hoses then you probably have a fair idea of what you're doing so it will be fun to work towards a solution.
Having read the other newish Renault threads on this forum, the idle control valve is the first thing to spring to mind. I gather it can get blocked, or can kind of stick. Apparantly it can be taken apart and cleaned up to make it work again, although sadly the article I was planning to point you towards seems to have disappeared. (Come on Chris - stop moving things around).
I'd be inclined to mess with that bit. It seems to be hella-unreliable.
Have you tried that? If that is the issue I'd be interested to know. If not, keep us posted with your findings and maybe we can help you figure it out.
:clementi: