I recommended 2 common codecs in a post above.
I asked you which you used in Vegas, in the other thread, to get some insight into your existing work flow.
You mentioned you work with HD footage. I was curious to learn what codecs are running smooth on your system... then it might be easy to select one that is relatively lighter for use with SONAR.
Are you working with Camera footage or footage that has been exported and sent to you from some one else's video edit suite? I'd like to know the actual codec... and I think I ask each time I've tried to help... maybe I can guess the codec if I can figure out where the source footage is from.
You mentioned a SD 720x480 widescreen export from Vegas. Was it the super light weight DV25 codec or was it one of the dozens of other high bit rate SD 720x480 widescreen codecs that are available?
Do you have any video acceleration on your DAW? If so is it just the "GPU" or is it a dedicated video I/O card from AJA, Black Magic etc.?
If you have hardware that is focused on decoding video than the CPU will have lots of room for SONAR.
If you use a codec that specifically tantalizes the instruction sets embedded in the latest system CPUs then you may find that the CPU starts focusing on the video play back while other apps get short changed. If the video or the DAW starts stuttering don't bother looking at Task Manager to see if the CPU had time to tell Task Manager it was real busy. The proof is in the experience you are having with the stuttering or black screen video.
A lot of the latest distribution codecs are meant to appropriate the CPU. The player apps play smoothly and they drop frames to make it look like it is all so easy.
The edit apps compete with the CPU intensive codecs and the timing gets sketchy.
The easiest way to deal with all this is to get used to the idea of knowing what codec you are working with so that you may compare an experience with it with experiences you have with other codecs.
all the best,
mike