Beating (possibly) caused by different sampling frequencies?
This is probably just a manifestation of your actual sample rate and your input wave not sharing a common divisor.

Here's a quick Matlab example, with a sample rate of 8Hz and a frequency of 0.92.

&gt;&gt; time=[0:50]/8;
&gt;&gt; f=0.92;
&gt;&gt; out=sin(2*pi*f*time);
&gt;&gt; plot(time, out);




It's not as dramatic as your example, but given time, and your funny resampling, I'm sure I can come up with something as dramatic.

Meeting Nyquist sampling criteria does not mean that simply plotting the data will yield perfect reconstruction-- it just means that it's possible to mathematically reconstruct the original signal.  Watch what happens when I up my input signal!!

&gt;&gt; f=2.3;
&gt;&gt; out=sin(2*pi*f*time);
&gt;&gt; plot(time, out,'-*')




At 2.3 Hertz, I'm sampling (8Hz) at more than three times the input freq, but the result still looks messy.

Resampling will only make things messier (but that's a hair much to spend time to simulate here, as Matlab resampling commands will upsample by interpolation, so doing it the way you're doing it is more complex)