Based on the limited information provided, my first guess is that your system simply can't handle the real time playback.
I doubt THAT is it, but since I am NOT a computer expert, and just in case that is the problem, what kind of PC info do you need other than what's below, so we can establish that this is, or is not, causing the image jerking to happen?
PC System Info:
Rating: 4.9 Windows Experience Index
Processor: Intel(R) CPU
Memory: 12.0 GB
System Type: 64-bit Operating System
Have you checked to see if it's jerking like that in a final encoded output? It may be happening only in playback in the timeline and it could be that your final output will be just fine. Try that, and it'll be a good test of whether or not it's your system or if you're actually having issues with your project.
You're right, and I'd thought of that and reduced the resolution on playback and it works fine. That's all it was.