Engine will not rev over 6k

I have a k20a2 swapped ep3 with an ep3 trans. It is kpro'd and tuned but it hasn't been dyno tuned yet. It's throwing a code that says a/f mixture too rich. I noticed today that the engine will not rev over 6,000 RPM. Vtec is working. The car is fine until it hits 6000. I have the rev limiter set at 8k. The tach readout is accurate. When I hit 6k rpm the car sputter quickly and bogs. The bogging seems to carry over into the next gear slightly. Any ideas?

Engine will not rev over 6k