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 http://osmc.tv/wiki/general/how-to-submit-a-useful-support-request/ 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 http://osmc.tv/wiki/general/how-to-submit-a-useful-support-request/