HD 1080 422
This is the format you say it was transferred to - is this ProRes 422, or Uncompressed HD, or something else? In either case, the machine you play it back on will have an effect on the smoothness of playback. It's hard to tell from your original post what exactly the issue is, but if it's a matter of the file being too big (uncompressed) or processor-intensive (ProRes), you might get some break-up of the image on playback.
For Uncompressed, you need to play it back on a computer that has a RAID array that can move at least 300-400MB/Second, reliably, to guarantee smooth playback. This means it won't play smoothly off of a single drive, especially one that's connected via USB, Firewire, Thunderbolt, or even eSATA.
For ProRes, the format is compressed, and requires the processor on the computer that's doing the playback to decode the image before it can be displayed. If you try to play it on an older machine (say, a G5 or an old iMac or laptop), or a computer without enough RAM, you will experience playback issues. Again, playback quality may vary from external drives. But you should be able to play a ProRes file smoothly from most external drives (Firewire, USB3, thunderbolt or eSata - but not USB or USB2), or an internal drive without requiring a RAID, if the computer was made in the past 5-6 years.
Converting to 720p, in either case, would eliminate most playback issues and allow you to smoothly play the files on more modest hardware simply because there's less data than with 1080p. That's why I think this is mostly a drive speed or processor issue on your computer.