Weird one, the engine is actually in a kit car and as the revs build to around 7,000 the engine power has a momentary loss in power (about 1/4 of a second) and then power comes in strongly to redline, its almost like a vtec?? Its weird, its not running rough or idling badly and the rev round once the kick happens is strong.
This momentary loss actually feels good as it boots you back into the seat once the power comes back on but I know it isn't right, its very quick, but its definitely there.
The engine has just has new NGK iridium plugs (CR9IEX), a dry sump fitted, fresh oil, serviced and a new air filter.
Could it be plug gaps? Coil pack?