I had this same trouble with a free tool I used to stick vids together... sometimes it would work and other times the image would freeze at the first join whilst the sound continued.
It wasn't terribly important so i gave up the idea.. but was later recommended this as a solution:
http://www.seller-club.com/~vcdcut/
I didn't try it myself but the person that recommended it seemed to know exactly what the issue was and was sure this would work at the time.. might be worth a look.