Well Im willing to bet its the video, because Star Trek, and Transformers played fine in 768p sized from 1080p through WMP.
(Perhaps its Windows trying to tell me not to use pirated videos, because the video im complaining about is like my only pirated video I have. Its called karma)
Years ago when 8MB was the big size for top end desktop hard drives, Maxtor had a Beta App that could set 16MB or 32MB of your System Memory to act as an additional buffer.
It really did work but was never made Final.
I've had bad videos as well. They're a fact of life, especially if you're using uTorrent to acquire them... The thing is to not go off blaming your hardware until you are sure certain the video is ok and the best way to confirm that is by playing it on a different machine.
Here's a good little short you can use to test true 1080p rendering on your computer...
Big Buck Bunny Download
This is a very taxing video to play... it's true 1080p, AC3 5.1 sound, in 32bit color... so I'd figure if you can play this you can likely play anything.
Moreover, if you are using Windows Media Player I would seriously consider dumping it... as in going into the add remove features dialog and unchecking it. A far better player that uses 1/10th the horsepower to accomplish the same thing is this...
Media Player Classic - Home Cinema - Video Player
Believe me, a few minutes with this player and I'm pretty sure you'll be disabling WMP for good. The only cotasil is that MPClassic does not work at peak efficiency "out of the box", you really do have to go through the settings and spend some time getting it tuned up for your computer. Once that is done... I think you'll be totally satisfied with it. It is the only media player that will render 1080p content on my son's Toshiba l300d laptop.
Anyway I hope this extra info is some help to you...
My guess would be that your cpu is being hammered during playback. What does the cpu usage look like in windows task manager when playing the problematic videos?
More likely it's win 7 itself... there are some terrible latency issues that are going to have to be worked out before it will do a good job on video or audio in low end environments. Things XP does effortlessly Win7 on a laptop or netbook, windows 7 chokes on.
It's about "dpclatency"... If you google the tool you will see for yourself. Win7 runs about 130 to 140 where XP runs 12 to 17.
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