Have you got a GRS or Pace I/C in there, if so I'm having the same probs. No gone to the same extent as you yet but will getting along those lines. Managed to split a hose which was sort of bodged on just to get me home and the temp hardly moved from the bottom so I'm very confused as to the probs with mine.

If you do have the GRS or pace are you running one fan or two. I've got only one at the mo but am seriously thinking about getting a second.

Sorry can't really help but would also like to know peoples ideas.