Some questions:

What did it sound like before you changed plugs
Did you change plug WIRES too? Cap? and if so,what kind of cap, brass contacts or aluminum? Rotor?
What kind of plugs? Fancy platinum quad contacts or regular old copper plugs .35 gap

Have you tested the spark coming from the coil? Snappy & blue...or just kind of half a yellow?

How's the oil pressure?

How do all your grounds look, greasy or shiny?