I'm pretty new to PowerDirector too, but I've learned that the preview window is best set to 'high' or 'normal' preview (they're middle to low settings). This mostly prevents the stutter in my experience. I assume you mean in the edit screen preview.
This preview is ok for most times when you're wanting to check that your music matches, syncs, that your titles work, the particles are good etc.
When it is not ok, of course, is when you're trying to determine quality of your piece. Try setting it one higher, then another higher, etc, until you can preview pretty high without it stuttering. Of course, you may be able to look at it in HD for a few seconds to determine that it's pretty good. Then it's best you drop down again, otherwise, things begin to play up.

It's one problem that is fairly widely known. And IMHO a problem which Cyberlink should REALLY fix. Pretty dumb having a preview screen where you can't preview the quality that you've shot in. However, you get used to it.
oh...one more tip. Before rendering, I've always had to make sure the quality of preview is still down in the med/low range. It doesn't make it render badly, but I've had some hiccups and best to keep it in the med/low for best render.

Hope I've helped.
This forum is so fantastic, the guys are great, and always help me.
I'm thinking they're all a bit busy right now with their new patch, and PD13 is so new too...there's lots to learn. It's doing my head in!! Jenny

Thanks so much jaymay22 - that did it! Wow! What a relief!

Wow -my first 'help'. Usually it's me getting the help. Jenny

Happy to be your first helpee! Best wishes on all your future PD and forum efforts!