I had a similar stumbling problem awhile back..it took a while to diagnose but it turned out to be my NEW (8k miles) Bosch Platinum spark plugs. The platinum electrode was GONE. Put in a set of NGKs 40k miles ago.still no problem.

What model Dynomax mufflers do you have? You said racing..are you referring to the Bullet muffler or do you have the Ultra Flow muffler?......I am looking for info about the sound the Ultra Flows have.