Fileout chop doesent work if not visible

hi there
i might do something wrong, but im not able to get the fileout-chop working when in performance-mode, it just records 50-80 frames and then i just stops to write the file!
when i attache a op-viewer to it and make the viewer visible in the interface it just works fine!
it looks like the fileout-chop needs to be visible to do its job right…
am i doing something wrong or is it a bug?
best
luca