HD 3200 with 720p

Hi, I have a laptop with a Radeon HD 3200 integrated (onboard).
I was wondering if that card is capable of running 720p quality movies on a 60 inch LCD Projection TV (tv supports up to 720p/1080i).
I tried to watch "The Dark Knight," approximately 8G in size and it's supposed to be 720p/x264.
And when I started the movie it lagged and stuttered noticeably.
But when I watch other 720p videos they run just fine.
Is it because of the codecs / graphic drivers / etc?
I use CoreAVC for codecs and didn't update the drivers for a long time (never since purchase).
I'm thinking it's because of the large screen size, but I don't know.
I ran 720p videos just fine on my old computer which has GeForce 6100 without any noticeable lag.
Any opinions would be helpful O.o.