In case of 23,97FPS Sources like Dvix or DVD mpeg2 you should never change the FPS choosing the hard way!
Stay at 23,97 and choose the PuldownFlagSettings in TmpgEnc:
http://www.kvcd.net/forum/viewtopic.php?t=5729
there it's explained how to do it correct and why we don't change to 29,97!
Maybe you found your 15FPS File in the www. Well these movies are just samples at low bitrate and low FPS just to rise up download speeds. They are not made to play them via stand alone players.
When you live in a NTSC country and your player doesn't support PAL Disks .... just convert your 25 FPS Movie by using Avisynths "assumeFPS(23.970)" command and slow down your Audio by using Besweets FPS conversion capabilities ... and create a 23,97-Pulldownflag-mpg as also explaimed in the link above.
By using these procedures you will receive well playing streams for your stand alone player and also this is the right way to obtain best CQ Values (also explained in the link above). I've never made a KVCD using a "real" 29,97 fps rate if the source is not based on a capture