Have you ruled out simple things like the distributor cap, rotor, wires and spark plugs? -- the fact that humidity makes a difference to how it idles makes me suspect an electrical issue. But this would not account for the evidence of fuel-rich exhaust -- are you getting unusually poor gas mileage too?

An easy thing to do that can cure rough idling is to remove and clean the throttle body with fuel injector cleaner.

I have a similar idle speed/roughness issue with a recently purchased k-jet equipped 240. Haven't sorted it out yet -- in my case the throttle body was dirty but not causing the problem. I thinks it's best to replace the wires etc. to rule these out before proceeding further.