Thanks Spook, good advice as always. I tried my back-up carefully cleaned original bosch iac controller and it was no different unfortunately. I had a pretty good look around the hoses as you suggested but no smoking gun really. I'm possibly going to try a new fuel pressure regulator with a some new vac hose as the next step. The car's been so good over the years it probably deserves some new parts even if they aren't the answer! I certainly think the poor old computer is having a hard time of making sense of whatever the problem is.

Will update this thread in a couple of days when the regulator goes on.

regards