You probably have a plug wire end bad. Very common especially after having replaced the plugs.
This is very easy to see with an ignition scope. Since plugs wires seem to be an acceptable throw away, I might suggest replacing all four. They will be less than paying some one like me to figure which one. Of course it could be something else, that's why we make the big bucks (bg).
The plug wire ends are all that ever go wrong with the plug wires and are easy to replace with a little technique. Here is my technique: first slice the rubber boot at the top of the plug wire end (look at your new end to see what's what) from the metal to the wire. Under it you will see white porcelain. Place a rag around this and squeeze it at the top with a pair of pliers till it breaks. Break it away (don't cut yourself the pieces are sharp) untill you can see the metal fitting made onto the end of the wire. Grab the metal with your pliers and unscrew it from the plug wire end. If you don't do this and try to unscrew it by grasping the wire the wire will almost always tear from the fitting and then you have a problem (fixable but thats another story).
Bosch Master, ASE Master, L1
33 years MB technician