Misfire - Check Eng Light Vibration

Hello All:
While on a recent highway drive, car began to vibrate with muffler noise. Soon after, check engine light came on. Mechanic read code as 3 cylinder misfire. He then inquired as to whether I used different gas... which I had. He suggested Supreme which I did, to no avail. Plugs and wires have been determined to be okay, as plugs are new platinum and wires are sound. Vibration is felt under load only and not noticeable while parked and running. When taken out of park with brake applied, you can hear an intermittent loping sound as well as vibration. Car is a '95 Crown Victoria with approx. 100k.

Any ideas would be helpful. Thanks.

You say the wires are sound. Is that a visual or did you check them with an ohmmeter? visual "looks good" means nothing on plug wires. I've gotten bad ones out of the box. If all 3 of the misfiring cylinders are on the same coil pack, have that checked. If all else fails, open the hood on a dark night and take out the bulb. Start it up and look for arcing. I've found bad wires that way. Check firing order. If you used Bosch plugs yank them. The only plugs that last in the modular V8s are Motorcraft from the dealer. Did you put dielectric grease in the boots? Did you detail (spray off) the motor? The way the plugs are oriented any water is just going to lay in the holes and cause a misfire. If all that fails, look at injectors - they will also cause a misfire code.

Replace the wires. I had the same thing on a 92. Visually the wires all looked good, no arcing was evident anywhere, but sure enough, replaced the wires and she ran smooth as silk.