This is a common problem for the little XR's (both the 50 and the 70), especially if they are stock. The best way to not only fix the hesitation but also improve the high revving performance is to swap out the stock CDI box with a good aftermarket unit like Takagawa. The unit not only fixes the flat spot you describe, but also removes the rev limiter, which is set at about 9800 despite the motor making power all the way to 12,500. The cost is about $60 but makes a big difference! Hope this helps.