You can fix the live preview flickering issue on XVR models JVS-XD25XX-FC10T and JVS-XD27XX-FC10T by adjusting the video standard of the device. Here are the steps:

XVR Preview Flicker

1.  Click the "Home" button on the lower left corner of the XVR GUI

2. Click "General"

3. Select the "Video Standard" to "PAL" then click "OK" to save the settings.