Catalytic converter? Timing belt slipped a notch?

Is it running roughly, or any other thing that may help to diagnose other than won't go above 4000?

Sudden onset immediately after plug change?
Gradual, more apparent now with new plugs?
Anything else done with plug change, like timing belt?
Age of timing belt?

The more information above "it doesn't work" the better.

2002 Sporty , Ironman 2.5" spring, 2" body, 15 x 7 ROH wheels, K&N, 15 x 10.5 Simex Centipedes, Powerchip 91.

"Never argue with an idiot. They will bring you down to their level then beat you with experience!"