Strange and random distortion from a very simple loop patch

Hi

I sometimes have with this very simple loop patch the strange distorsion seen in the audio attached. I can't find the reason why : cpu load is 10 or less with the all patch, works great rest of the time. Intel 2.4 with 10.5.6 but tested with a small Aureon Dual Usb sound key right now.
Any Idea ? Or more accurate peace of patch to do this ?

Thanks a lot

Charles