Glitches after render

Hi there,
ive got some really bad glitches after rendering my timeline. Im using custom luts/bought from Matti H but whet it comes to glitches I doesn't matter color correction...I can just disable sharpening or color curves and glitches disappearing for a moment - after another render they came out again.

Any help from you guys?
Im using iMac 5K, i7 4.2, 40gb of ram, r580 8gb. Files are located on external usb-c nvme drive. Files are from Sony A6300 but it doesn't matter. Files from Canon 60D glitches the same

What specifically does "glitch" mean?

I mean weird colored squares in the preview. Sometimes they are bigger, sometimes smaller and in different clips - and after always rendering. iMac was in servise and Thel didnt see nothing wrong with gpu etc

That's just kind of what it does with a few certain plug-ins, but usually when you export it should look perfectly fine regardless of if you've rendered or not. So try exporting a segment just to make sure.

Are you using a custom LUT tool?

Not really...after export the same things are in final clip : ( Its very frustrating

Yes, but things are happening needer its Custom Lut or mLut Loader. Don't know why...after glitches show up I can for example disable sharpening and they disappear - for just a moment...after render they still there. Just went back go FCPX 10.4.6 - maby its new METAL engine