Hmmm, maybe the gfx are lagging out the brush? I'd try setting the main draw window to wireframe for a test stroke and see if the problem might be something fpu-related. WHen i had a bad gfx driver installed i had problems with loads of the brush related functions, also editing terrain. I had no problems in wireframe mode though and when i set resolution to something low like 1024x768 it was much better, so i found out my gfx card driver was a version which was way too old. After updating it i didn't have any more problems. Not sure if that also helps in your case though... 

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