This is a problem in a 1997 Taiwanese DOS game called Sanguo Fengyun (三國風雲). This is the original DOS version of the game, not to be confused with the stand alone expansion Sanguo Fengyun: Qijian Ban (三國風雲 旗艦版) that came out a year later running on Windows.

The title screen of the game runs in 640x480 (with a black bar along the bottom??) and has horizontal black lines on the side:

In the game the default mode is a low res mode that works fine:

However the game has what appears to be a 640x400 (no black bar on the bottom) high res mode (accessed with the F4 key) that is far more desirable for play, but it has the same black lines issue as the title screen:

I am using Dosbox 0.74. I have tried all the different machine display modes for SVGA, as well as all video output modes. Sound in the game works fine.
There is also a problem with the games cinematic FMV scenes where they crash dosbox, but these can be bypassed by mashing the ESC key to skip them before the crash. I dont really care about these and just trying to get the black lines issue solved is first priority, so I can enjoy playing the game in the high-res mode.

Thank you very much for any info or help!