i rendered a hi-res MP4 24-min video from a VP12 timeline. on two different occasions, about 2min of constant obvious flickering is happening. it's at or near the mid way point. any ideas offhand?
I had the same issue in V11. Try to turn off GPU acceleration, however in my case mix of GPU = on, 296.10 version of nvidia GTX drivers and dynamic RAM preview = 0 was the solution.
The problem may be also too big resolution of image that you put on the timeline.