I *just* replaced the plugs on my '99 VC XC, and it ran fine for 20 minutes. I put back all of the plug covers on the engine, then took her down the block for a spin. As I left the driveway, the car threw a CEL, and was running roughly (car shaking).

The problem was cylinder 4 was misfiring. Long story short, when I put the covers back on the engine, I has accidentally crimped and exposed the wire going to the coil pack on cylinder 4, causing a short. This caused the coil pack to not fire, causing the CEL. I electrical taped it, and carefully put it back together, and reset the DTC with an OBDII code reader, and all is fine now.

Seems to me the plastic shroud used on the '99XC is pretty much brittle and destroyed (I had to vacuum a lot of this crap out when replacing the plugs) - it's on my list for replacement in the near future, when it gets a little warmer in upstate NY. :)

Inspect the wires under the covering, make sure you didn't tighten something causing them to short.