ive been having this problem for a long while and ive been looking around for a solution. and I found a very similar question explaining my very same issue. I edited it so i can give you all more details. unfortunately i dont know how thats going to work. so i decided to post it again in a new question hoping for your help.
"I have a new 15" MacBook Pro with the highest specifications, running Lion version 10.7.2. However, when I watch movies they sometimes lag on my computer: frozen screen (for 3-4 s) with lots of broken cubes and with the audio track still playing). What could be possibly wrong with my computer?
For example, today I watched the Fast Five Blu-ray movie (which is almost 50 GB) with the VLC media player and suddenly it started lagging on some episodes (especially on intense action ones with lots of special effects). Moreover, it lags not only on Blu-ray movies but movie files ripped from standard definition DVDs and other formats as well.
I play all those movies with the power plugged in from downloaded files, not from optical DVDs. Even Stanford University lectures from iTunes sometimes lag for me.
UPDATE: I was in the Genius bar several times, reinstalled Lion by my own and also in the Apple Store.
After the fresh install I tried to test videos. I watched House, M.D. season 8 episode 2 (395.9 MB) yesterday, and it started lagging all over again. I do not really know what to do. Is this a bug in VLC, iTunes, and other media players, or it is something wrong with my hardware (they actually tested my Mac in the Genius bar with their cable plugged in but did not find any problems)?"
Ive also tried using different software like Movist and Miro but yet,I have the same problem still. it almost always happens on intense scenes but when i rewind it and play that same scene over and over it works eventually! so i dont think the problem is from the movie file itself. i noticed a few things though: -1 it happens with youtube videos as well regardless of which browser im using. -2 it NEVER happens when i connect my computer to an external screen using an HDMI cable!
I had an issue playing HD videos on my older computer and its the main reason i bought this macbook pro. so you can see why this issue is very frustrating to me.