A better definition of "choppy/blocky" would help, possibly even with a single frame screen capture to show what you're describing.
If the playback stutters, it can be for a variety of reasons - if it looks like long pauses it's probably dropping frames (which it really shouldn't on a MacBook Pro).
If it has what appears to be odd quick stutters at full screen there's a possibility that it's a frame rate rendering issue (ie: if a movie is 1080p24 (24fps) being played back on a screen that has a 60hz refresh, it can cause what I like to call micro stutters due to the refresh being out of sync with the fps of the video being played back)
If it looks like there are large block squares in the picture (similar to what you might see in the pictures here:
The good, the bad, and the…beautiful | TokBox Blog) it could be a compression issue - H.264 is a lossy compression, the more highly compressed a file is the more "blocky" certain areas (especially what might appear to be constant tones like black or blue sky where there really are many gradients) will appear - also motion will suffer from this with higher compression. All movies digitally encoded will display some of this - Movies from Apple for example are very highly optimized and look very good for their overall size but can still show some "blocks" in certain scenes/backgrounds due to the compression (think about it - a 1080p movie on bluray can be anywhere from 20-40ish with the average at about 30ish mbps for video. Most 1080p video files are encoded at a significantly lower bit rate - quality at that point will depend heavily on the algorithms used as well as the ability of who is doing the compression to optimize the process for the source media (which is why movies on iTunes usually look very good for their respective size)