how to make displacement in cycles to work?

I use cycles and displacement output, with mesh displacement type = “both”.

Sometimes, under hardly determined conditions, blender starts to ignore displacement completely, or to use some improper cached UV map for texture,
untill i create fresh new file and import all the objects and materials in it.
That sucks alot and i start to hate blender.

Do you know how it is possible to turn displacement back?
Maybe some internal caches to clean or whatever?

Cycles displacement is still experimental, will be fixed in an upcoming release (2.67 probably).