RM250 popping under acceleration, power delivery not smooth

My friends cousin is a professional rider in sweden (his name is Ludwig) and has a bike here in America for when he visits, but since he is only around for about a week a year he lets us use the bike for the rest of the time. We both have bikes of our own and have grown up riding, but this thing is just brutally fast. It appears that nothing is stock on the bike from the handle bars, triple clamp, clutch and brake levers, sprockets, chain, full RG3 suspension to the aftermarket clutch cover, shifter, bills pipe and silencer, to various other billet nuts and bolts littering the engine. We ride it on the road frequently, and love to open it up as often as possible, and have never had any reliability problems. However, since the first day we rode this bike it has always had this problem: The bike pops under acceleration and does not accelerate smoothly at all. It is almost as if during acceleration the power cuts on and off very briefly until it hits the power band, and then this thing takes off like nothing I have ever ridden before. we clocked it along side my friends audi at 90MPH possibly more going up a very long and very gradual hill by our house. So keep in mind that it does run well besides this problem. I would just love to feel the acceleration this bike is capable of without the blips in power delivery under acceleration. Any advice is welcome, and thanks for reading this whole thing.