sounds a lot like a low speed vac leak, a biggie. THat will give you a high idle, with probably a miss if it's in one cylinder runner, generally crappy lean idle and off idle (until the air flow rises high enough the leak is masked by volume).

A leaking intake changeover valve shaft will do that kind of leak, and massive engine damage may be close behind, so it's a must check. It's the thingy under the cover that is a vac pot linked to a crank arm linked to a shaft running front to back thru the lower intake runners. Do you hear any fluttering sound at cruise speeds?