If you can get a good look at them at night (while its dark) you may be able to see sparks coming thru the wires while the car is running. Once you get the wires off (shut off the car first!!), look for little white rings on the wire boots. This could be a sign of the spark coming thru and shorting on the engine block instead of lighting up the plug. Magnecore's website has a good set of pictures and a good description of this problem. Also look for corrosion on the connections. If you see any of these, the wires are bad. The plugs should be clean of oil and crust etc...
Also, check your vaccuum lines. If your car cant find gears easily and its stumbling goes away when you floor it at high rpm's this could be your problem too.
Hope this helps.
