Some missing frames in ripped DVD

Hi.

I've tried ripping two different DVDs, each twice with the same result:
The frame rate is not consistent, there is 1 out of 5 seconds when the
frame rate suddenly drops (from ~29 to ~15). I could see the movie
jittering, the effect can be observed more clearly if there's some
objects moving across the screen.

Is this a problem with dvd::rip's back-end or mine ?. Most of my
encoding settings were unchanged. Btw, I encoded them with "-u 20,2" and
"-u 4,2", my CPU is Intel Core 2 Duo.

I will try it without the -u option if I must, but it already took 10
hours for one DVD.

Thanks.