At tick over I have a surging problem.

I bought this Cadillac with cracked heads. I have replaced the heads and have the engine running and starting perfectly. I replaced the distributor cap, rotor, wires and plugs. Runs well with alot of power when revved up. When idling (ticking over) the engine surges. I had it checked on a computer and it says that there are no faults. Any guidance would be appreciated.