Is the cga mode still broken with machine=cga?
Did you set machine=tandy when testing the tandy modes? If so, try
machine=pcjr as well.

Yes and yes. When I first tried the game it was with machine=cga... the scrolling has been broken under every DOSBox version I've tried (since 0.61). If I remember correctly from my childhood (heh), on an actual XT/CGA machine the scrolling used to be very smooth AND fast... could be a fancy hardware trick or something.

When I set machine=tandy, and select Tandy in the game's own startup menu, it still gives the same CGA graphics and pc speaker sound. That's why I think it might be a missing game file - I remember someone here posting about a similar problem with Thexder (solved when the full file set was obtained)... but ALL the copies Prohibition that I've managed to find have the same files that I do, no extras.

Tried machine=pcjr??
Is it sure that the game worked on a tandy?

Same for me here. Using PCJr mode. I believe Tandy mode is the same as CGA in this game (it's there just for compatibility reasons, but who knows). It also supports special CGA mode Amstrad PC1512 had (which if I recall correctly had 16c in low resolution). Anyway the scroll is broken and game cannot be played.