hi lachlan,
if you're parked on the problem frame, does purging the memory cache fix it? if not, and if the disk cache is enabled, try disabling/emptying the disk cache, then purging the memory cache. does that fix it?
are you on 11.0.2? it fixed a few caching-related bugs.
is it the roto brushed footage or masked footage that's causing the problem (i.e., if you mute one or other does the problem no longer appear)?
if neither fixes it, would you be able to send us (
aebugs@adobe.com) a simplified project, and w/ sample footage (if it's related to roto brush)?
thanks,
:jeff