Hi Carl,
I have had one with a very similar fault which was previously an airport security vehicle wih 40K on the clock but had probably been run 24/7 for 4 years. Main dealer fitted a turbo which made no difference and I fitted a MAF as this one also ran better unplugged. I took off the inlet manifold because it was pretty choked up and cleaned EGR etc. I was convinced it was a breathing fault but never got round to cleaning the exhaust manifold as the customer felt that it had improved enough for him to drive it. He has now had it 3 years and drives it for "normal" journeys with regular Forte treatment and although it's no powerhouse it is acceptable.
HTH's,
Bryan.