Idle speed while in park is around 800 but when in gear and at a stop it's 600 or even 500 and when in gear and at a stop the economy gauge is 1/4 of the way in which my 560 SEL doesn't do at all.
That suggests a vacuum leak, doesn't it?
I've heard of fuel injector seals leaking air badly enough to produce rough running in K-Jet cars.