Fx disapear

After working with fx files, sometimes, randomly, and with no reason I can figure, they stop working, the renderer goes black, and if I delete the fx node and try and load it again, it go3es through the motions of loading but no node appears in the patch. On shutting vvvv down it take a little while then a vvvv has crashed error pops up, I have to reboot to continue working.
Think thats it!