sculpt, some glitches

hi,
I’m still sculpting my boots,
http://en.zimagez.com/miniature/28607.png
the mesh has 538076 tris. I subdivided the mesh several time because I get some glitches when I sculpt on it. Even with 538076 tris I still got these glitches,

http://en.zimagez.com/miniature/29544.png
in red with no smooth, in blue after smoothing. This happen only in some parts of the mesh… it’s really annoying!

why I got this ? what can I do to solve this ?

Insufficient mesh resolution.

Add additional divisions in the multires modifier or use dynamic topology option in the sculpt tools

if I subdivide more, I get some lag when I sculpt… dynamic topology works fine, so I will use it…

thx