Developer Forum

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
to Normal.

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

[snip]

A big image would slow it down, but should not stop it completely.
Perhaps you were close to filling RAM and the Undo information
required enough more memory to make Windows start paging to the swap
file. If it happens again, try waiting it out for a couple of
minutes. The strokes might catch up.
--
Fred Hiltz, fhiltz at yahoo dot com