Upscaling 4k Glitch artifact

Hello,
I'm trying to upscaling old video and I'm facing trouble in exporting the video:
If I upscale to 2k, no problem. If I do it in 4k, there's compression artifact, as show on this caption




Most of the time, these problems occurs on a black screen after a simple cut sequence.
Is there a 100% safe way to avoid these glitch?
(Sorry if there's english mistakes, I'm french and my english is a bit rusty.)

Try exporting to something like Cineform or DNx as a first step, then encode that to H.264 as a second step.

thanks Jim, it worked as you said.

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