Looks like an amazing game but there is very noticeable vertical jitter on Stella 6.7. The line count in the game alternates between 262 and 265 standing still and varies wildly up to 270 when jumping in the air. Line count issues can result in an unsteady picture in an emulator or on digital displays. On a CRT it may look fine but they are much more tolerant of issues like this.
If you run your game in Stella and press Alt-L you'll see the line count in the top left.