[PD] sound with glitches

[altern]

hi

sorry if this has been asked before. I am in the middle of a workshop 
and one student is having problems with the sound in PD, this is both 
with the latest extended version and the latest miller build

he is running a
ppc g4 167gz on OSX 10.9

he is just playing a sine wave but the sound is dirty and periodically 
distorts (ones per sec or so). We tried not to load any externals, also 
increased the sound buffer, restart the machine, restart pd few times 
... but keeps making the funny noise

i was wondering if anyone has experienced anything similar. we tried 
supercollider and MAX/MSP and both work fine.

thanks!

enrike