DVD subtitle problem

[dvd__r]

After a painstaking process and spending MANY hours with Tmpgenc, I managed to convert an Xvid AVI file to DVD compliant format. The video looks great and the audio is pretty good, but the problem I have is that the accompanying subtitle stream is viewable until a certain point in the movie and it just disappears. Using the subtitle on/off option in the DVD navigation after that point doesn't help. Anybody has any idea what's going on here?

My conversion process is:
Video:
Xvid AVI --> Tmpgenc plus 2.5 --> *.m2v
Audio:
Xvid AVI --> Virtualdub 1.5 --> *.wav --> AC3machine/Besweet --> *.ac3
Subtitle:
*.sub --> Subrip --> *.srt
DVD authoring:
IFOEDIT muxing all files: *.m2v + *.ac3 + *.srt --> *.vob, *.ifo, *.bup
DVD burning:
imgtool

The resulting folder VIDEO_TS on the PC can be played in WinDVD without any problem at all for the subtitle. It's only when played on a stand-alone player (Hitachi and Samsung) that the subtitle keeps disappearing, at a specific point. By the way, as you can see from above, the subtitle is not permanently encoded with the video. I've tried it without much success though as Tmpgenc keeps crashing on me when I raised Directshow priority for Vobsub to work. But that's a whole other story.

[dvd__r]

I missed showing one more step in the subtitle conversion:
*.sub --> Subrip --> *.srt --> Srt2sup --> *.sup

and IFOEDIT takes in the *.sup file, not the *.srt file as mentioned earlier.