Check for condensation on the spark plug insulators. You replaced the plug wires, and state the rpms are steady. I'd bet the rpms are not steady, but the signal the tach looks at is steady. You may have bad wires from the factory, as you don't state if the problem came up after swapping the wires. I added a problem once by replacing a coil that I thought was on its way out with a new one. What a pain chasing a problem caused by new parts.
tom