I was fairly astounded to see the difference in spurious signal generation between the two versions of the WsprryPi .  Here’s the output from https://github.com/8cH9azbsFifZ/WsprryPi:

And here is the significantly cleaner output from https://github.com/JamesP6000/WsprryPi:

What caused the reduction in spurious outputs?  I’ve been studying the programs, and while I still haven’t figured out a lot of the details, one significant difference is the addition of this bit of code:

while (n_pwmclk_transmitted<n_pwmclk_per_sym) {
// Number of PWM clocks for this iteration
long int n_pwmclk=PWM_CLOCKS_PER_ITER_NOMINAL;
    // Iterations may produce spurs around the main peak based on the iteration
    // frequency. Randomize the iteration period so as to spread this peak
    // around.
    n_pwmclk+=round((rand()/((double)RAND_MAX+1.0)-.5)*n_pwmclk)*1;
if (n_pwmclk_transmitted+n_pwmclk>n_pwmclk_per_sym) {
n_pwmclk=n_pwmclk_per_sym-n_pwmclk_transmitted;
}

This code snippet is located at line 300 in this file: https://github.com/JamesP6000/WsprryPi/blob/master/wspr.cpp

There may be other relevant changes to the code, but this looks like the big one when it comes to output spur generation.  I will play with it, as well as digging into the rest of the code, and report back.