1997 Ford Thunderbird smokes and check engine light

1997 Ford Thunderbird mileage 122,000. Last week my T-bird, (mechanically excellent), suddenly started smoking a lot (super rich, actually had gas drip out the tailpipe!). The check engine light came on of course. I took it in and the mechanic finally identified the excessively rich condition as a fuel injector that was hung open (after he replaced a water temperature sensor that OBD said was bad.) They replaced the injector, which stopped the smoking, but now it has a random misfire. They changed the plugs, guessing maybe one had a cracked porcelain and they said that fixed it. When I picked up the car, about halfway home (fifteen minutes of operation) it started misfiring again. Badly, but it was totally random. Sometimes I'd start off from a light and it would barely move, the next time it ran perfectly normal. It was like someone was throwing a switch, turning half the cylinders off and on. I pulled over and popped the hood. It shook at idle as if one or two cylinders were down and the suddenly it would hit on all eight, again totally random. While driving, it felt as though it was randomly dropping half the cylinders. When it wasn't misfiring it was running great. It really feels like an ignition problem. I tired pulling plug wires one at a time, but it was missing too randomly to figure out if it was a bad wire or what. Now I'm thinking a bad coil pack. The ignition module is in the computer (which was replaced last fall) so I know that's not it. The odd thing is the misfire didn't start until the car had been driven for fifteen or twenty minutes. Heat/resistance seems to be a factor. Could it be bad wires? Maybe the carbon was brittle and got cracked when they pulled all the injectors out. Any ideas?