Yes, converting a movie from NTSC to PAL or vice versa can cause this jumping due to the difference in frame rates (PAL is 25fps, NTSC is 29.99fps). Ideally you would preserve the video system when converting. First check your DVD to see if it can handle both formats, in whcih case you know you can preserve them.
To find out what format your originla file is, download this great little app
http://www.headbands.com/gspot/ which will look at your file and tell you all kinds of audio and video details aboout it. Just have a look at the frame rate to see what system it uses, then yoou know what to specify in NERO to preserve it.
Unless NERO can tell you that info of course, but I'm not sure....