One thing that can causes this is a brush variance parameter set to
Fade Out. View the Brush Variance palette and set all the parameters
Less likely, but worth checking, is the presence of a selection that
you cannot see. Next time it happens, try Selections > Select None.
While you are on that menu, if Hide Marquee is checked, click it to
make the marquee visible.
Fred Hiltz, fhiltz at yahoo dot com