Jerky video playback in the Preview monitor is usually caused by a under-performing computer CPU. Apart from DV-AVI, which requires very little processing power, most video codecs are requiring increasing CPU power to process. In the case of HDV MPEG 2, a dual core system equivalent to 2.5ghz is necessary to get smooth playback. AVCHD requires an even faster CPU. Playback smoothness will also depend on the effects applied to the object/clip. Effects, depending on their nature, require extra performance to process and may result in jerky playback when the original clip played smoothly.
There are a some things that can be done to improve playback performance:
Help has a reasonable explanation on the Playback screen: Search for "cache".
Note that I have had trouble exporting a cropped video with other than "Full Resolution" checked: the resulting video had additional cropping applied.
For more info on this screen, search Magix On-line Help for "Video/Audio".
Compatibility Mode (VideoForWindows)
Provides good performance for HDV (eg 1440x1080i) files. MEP uses the CPU.
Hardware Acceleration (Direct 3D)