remux VCD 2 SVCD with 2 audio files >> original audio gets async

[Tyler]

hi, this is a very strange problem.

1.) demultiplexed the vcd mpg (tmpg and mpeg-vcr) ntsc 29,976fps
2.) bbmpeg: multiplexed videostream with demuxed audio plus new audio stream as SVCD
3.) original audio is sync at the beginning but the delay gets bigger and bigger from time to time.

tried several methods, couldnt find a solution.
any hint?

thanks in advance