ZBrushCentral

Brushes stops sculpting!

Hi:
I have a entire model and lasso the hands and zoom it, i start to sculpt over it but in few minutes the brushes stop to sculpting, Why?
I have the model in 6 subd, and i do this in the 4 divi.
I install the last plugs for the wacom, and i have 8 gigs with windows 7 64 bit, put the mem at 4096 and only 4 undo docs,clear the masks, too i tryed some things but nothing…the brushes are claytubes and standart in zbrush 3.5 r3.
thanks in advance.