Hopefully this will help someone - Erratic and unpredictable render results

Question

This may be a known anomaly, but a quick search in the forum didn't return a hit for me, so I thought I would post my experience for anyone who may be experiencing the same thing.
For quite some time, I have been experiencing unpredictable and erratic results when rendering selected clips in a timeline. I shoot and edit 720p30 and 720p24 formats. The erratic results presented themselves when playing, although scrubbing through with the play curser looked fine. The results included freeze framing, flickering between clips, etc. I could never duplicate the problem twice in the same spot on the same clip. Sometimes, even previously rendered clips that were playing correctly would become “corrupt” when a render took place on the timeline. It seemed that it may happen with more frequency on clips with motion blur and time warping, but I had no data to back that up. It seemed that when I noticed it happening, I could activate the system monitor and see that "used memory" (the blue slice) had completely filled the 8GB pie chart. It was extremely frustrating.
Finally, I noticed a correlation with the render anomalies and one certain setting. Most of the time I edit, I am using an external HD monitor attached to BlackMagic video out (you can see my complete system config in my profile). Occasionally, I wouldn't use the monitor and set view>external video>off. I noticed that when I have view>external video turned "off", I NEVER get the render problem. After noticing this, I tested it extensively and always got the same result. So, now I use the external monitor for color grading in Color and for final review of the timeline. When editing, I just remember to turn external video off and use the FCP canvas. So far, the work around has worked flawlessly.
Although I still do not know what the reason is, if you are experiencing the same render anomalies and have view>external video set to "all frames", try turning it off, delete your render files for that sequence, render again, and I'll just BET your problems go away.
Hope this helps someone.

Answer 1

Wow! This is E X A C T L Y like the problem I've been having. I've sent through a few posts asking for help and although I've received plenty of help, this is spot on the button. Thank-you, thank-you! I've just tested it and your workaround works!
I also thought it was a corrupt clip and have tried to repeat it but no luck. However, I have found that when I switch off the edit monitor for a viewing for client approval, the flickering you described happens in my canvas. Just done a test with that and gone view>external video>off and I don't have to restart. Thanks for the help!!
Cheers,
Susan.