I have had that problem with a leaking "candle" boot.. but you said you'd checked the wires.
It's really hard to diagnose this problem from a continent away, though.
Only starting with the throttle open a bit is normal. Not idling, of course, is not. Coughing and unevenly firing, combined with not idling "sounds" like a vacuum leak.. but maybe not.
My guess is you will find your problem there.. but maybe not.