Another think you could try: Instead of calculating the rate of the Impulse ugen in the synth, use a Select ugen to choose between three different Impulse ugens running at rates 50, 100 and 200. Send the ‘which’ value from the pattern.
Or maybe better: put three different PlayBufs with the appropriate Impulse ugens inside a SelectX.ar and use a small lagtime with the ‘which’ value to crossfade between them.