No video

Using RASP2
sometimes, start playing a movie, get only audio, but black screen.
Problem clears by re-boot,but happening frequently. OSMC fully updated.

To get a better understanding of the problem you are experiencing we need more information, including logs from you. Please see for advice on how to help us help you.

I’m also getting massive lag while no video plays. Trying to play MKV files, they were fine with july version… Raspi 3 with latest osmc version. How to downgrade?

There is no downgrade path (other than installation of a previous version)
So the path forward is to fix any problems that may exists. If you want that to happen follow the instructions that @ActionA posted

As requested previously, we need logs to get a better understanding of the problem. Please see