This description sounds rather like a problem I had back in November - I wonder if it’s the same thing?
In my case, playback was quite impressively messed up - it looked as if it as actually displaying some of the frames in the wrong order.
But, like you, I found that skipping forward, or starting playback from somewhere other than the start of the file, made it play smoothly.
@LrQ9Sxkh0JTx , it might help the guys to diagnose your problem if you could upload a short video clip that reproduces it - something about a minute long, maybe. You could use something like mkvtoolnix to produce it, and then use some filesharing service (WeTransfer.com for example) and post the link here.