Please see the following link:
https://youtu.be/48tP_OlYrxc

This video is an example for a problem I having when using "VIDBOX Conversion for PC" for converting my old VHS tapes to files.
As you can see in the video, it starts normal, but from moment 02:34 the footage becomes leggy/choppy, than reverts back to smooth-play in 04:35.
In long conversions of tapes, this condition goes back and forth every few minutes, in random moments for each conversion.
The VCR appliance seems to work fine when connected directly to the display. Also, the preview does not show this while converting.
I have tried all the parameters in the software and also tried it on several machines (including a strong PC with advanced graphic accelerator).
I use VIDBOX software while no other application is running and also disabled background applications and services (as much as allowed by Windows). Also tried overclocking the machine, using enhanced performance options and tried different resolutions.
Please help me.