Hello everyone

Need help with my 99 Maxima GLE. Weird behaviour, dont know what to blame - engine, transmission (auto). At exactly 60 km/h if I start playing with acceleration pedal for about one third the car shakes and vibrates (like really hard shifting) pritty bad, the same arround 90 km/h. Dealer di not find anything, no codes. 99500 km on it, just reflashed tranny but the problem still their. If I accelerate slowly this is not noticable, shifts smoothly. Another simptom is the RPM gauge stics (I hear the engine reving but dial stays in plays and then suddenly releases and shows normal).
Any suggestions please.