A graphics card has to buffer data. It loads so much then will begin to play, how much it can store depends on the RAM of the card. If you are experiencing stalls or slowdowns like that its likely you have a card with little RAM for what you are trying to push through it. There are more technical aspects of Graphics cards and GPU in regards to this but it ultimately comes down to “Card Speed” in the end and that’s simply how fast your card can process data and how much it can store for viewing on screen.
Try rendering out an SD version of the same video. If your current one is HD. If its already SD then just try rendering out a smaller version. Then see if you get smoother play back. If so then I’d go with the graphics card/GPU speed and could also conflict with QT player and make it worse but I think its a hardware issue.
Your graphics card can only store so much data before play back on the monitor but if those pictures to display are smaller, it can store more of them and you will get a smoother playback if its the graphics card.
