Could a header leak at the motor cause an intermittent miss.

Can't say I've ever heard that causing a miss. I'd check the wires.

Leaky header can cause a lean condition but I would really check for a bad plug wire or fouled plug.

You sure its a actual miss?I added "Heat" one time cause couldnt find nothing wrong..it fixed the ood unsteadiess..water in gas..also the best thing to do is pop the hood at night and look for any grounding plug wires.sometimes its not even at the head it could be between two wires that lay on each other.