Interlace vs progressive DVD problem...

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Hi everyone!

I am currently authoring a DVD with DVD-Lab Pro. There are some photo slideshows and videos that I created with Sony Vegas.
If I play the output DVD on my PC, everything plays fine.

But here's the problem :

When I play it on my HDTV (480p via DVD player plugged with component cables), my video plays fine 95% of the time, except for two 15 secs segments where the image starts to shake a little.
Now, if I play the same video on the same DVD player but connected with composite cables, the problem is solved. Can someone help me with this please?

I have the feeling that the problem might be caused by the DVD being played in progressive mode on my TV vs being played interlaced.

I encoded the video in Vegas with DVD NTSC settings (interlaced). I tried to switch the settings to DVD NTSC progressive video but it didn't solve the problem.
Also, both segments where the image is shaky include a white background with some .psd files zooming in... I don't know if it's a coincidence or...
I'm not very familiar with all the interlace vs progressive concept, but I'd like to have a DVD that plays fine whether I play it on a HDTV (progressive or not) or a good old CRT tv. Should I encode the whole video in progressive or interlaced mode? And why is the image being shaky sometimes when played in 480p?

Thanks a lot in advance guys!!

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...also forget to tell that the 2 segments include .psd files with some transparency level...