[thstone]

A flat spot at 6K rpm is definitely not normal. And rarely (never?) will a performance modification eliminate a foundational problem (more often than not, the upgrade exacerbates the problem).

Check the ECU for any codes (like a misfire) that may have been detected but did not illuminate the CEL. After that you'll have to do some detective work to sort through the usual suspects like plugs, fuel filter, air filter, fuel pump, coil packs, MAF, etc. to find the culprit.