basic sawtooth

Hi, I have a simple question for all of you...
I am trying to program a sawtooth wave, with the k5m audio output connected to a oscilloscope.
 With the first 4 harmonics, everything goes as it's expected (simulated with mathcad and done manually the fourier series).
But when I add the 5th harmonic, the output it's way off wrong, and the same happens with 6th and so on.
To add more confusion, when i see the partials alone, they are ok...a 110hz fundamental results in 220, 330, 440, 550, etc etc...
Am i doing something wrong?
 Anybody has done a similar test?
 

 If required i can post photos of the output...

Thank you in advance...Nicolas.