and check your disks and OS for trouble before proceeding.
I should explain why I mentioned this... One possibility that could explain that kind of behavior is if the disk is too slow, or too messed up, to serve the video file fast enough. This would be particularly true if the file has a high bitrate.
A good test, since you have a SSD as the system drive, would be to copy a video file over there and try again. If the problems vanish, then you know it is disk performance in some way that is causing it.
External disks (particularly older USB2 ones) may not be fast enough to provide adequate performance for video playback. It IS possible (I do this fairly regularly with USB2 disks), but you're always going to be running "near the edge". There are also other things that can degrade USB performance, including other devices attached to the same bus (which can slow down all devices connected to the same root hub), driver issues, or just if the volume is quite full (or fragmented).
For the record, ROST should support video playback on even very low-end hardware. Some high-quality sources may require a more powerful machine, and it is impossible to say what might be wrong without further details on this particular source. But, I think an Ivy at 2.5Ghz can almost certainly handle "normal" 1080p sources in 1080p.
I have a Core2Duo system here that can handle 25mbit H264 1080p videos all day long without trouble in ROST. It can't play them at all in ROHQ, of course, but unless it is something like a raw BluRay rip at 50mbps then I'm skeptical that it is a general performance limit of that machine.
But... Again, all of this assumes you aren't using Windows Server. If not, then I'd guess that it is likely to be covered in the articles I linked above (anti-virus interference, disk, drivers, or DirectX trouble).