Question Random frame jumps in a rendered 4k video

Hello. I tried rendering a 4k video composed of several videos I shot in 4k. What I got was a horrible jumping frames mess, with some old frames repeating randomly and ruining the whole video. I tried different settings and lowering project's FPS (my videos got about 26-27 FPS, so I used 25 for the project)- but it only made it worse. In preview window though everything is fine. What can it be and how do I solve this? Thanks in advance
My Movie Studio version is 13.

Hi Plummet

I assume you are using Movie Studio Platinum 13 ?

What is the original Frame Rate of your 4K videos ?
Please download Media Info tool from here:
mediaarea.net/en/MediaInfo
Open one of your videos, then take a screen shot from the Tree View and upload to the forum, so we can see the properties of your video.

For 4K video to work properly in Movie Studio, you must make sure it is set to produce "easier to play" Proxy Video versions.
Go to Options/Preferences/Video
Automatically create video proxies for Ultra HD video, should be turned on.
When you import each video, the program will render the proxies.

You must also make sure the Preview Window is set to Draft or Preview quality, to show the Proxy versions while you edit.
If you have the Preview Window set to Good or Best, proxies will be bypassed and large 4K file versions will play instead. This will cause HUGE lag while editing, so make sure to set to Preview quality while editing and you should notice a difference.

I would still like to see a readout of your 4K file properties before commenting any further.

Regards
Derek