Well, when you replaced your plugs, did they all look the same, or were some more fouled than others? Did you change the plug wires too? Use OEM plugs?
Well, I would put a spark tester on the end of each wire, one at a time, just make sure you have a healthy spark to each plug.....
Other possibilities are compression problems in a cylinder(s), bad bearing in the distributor, bad injector, vac leak, etc.
Have you scanned for codes?