I recently just got my very first Apple II, a //c ROM version 255 hooked up to a floppy emu. The computer turns on, seems to boot fine, AppleSoft works etc. I can run some programs fine like Oregon Trail but others either crash straight away like every version of Merlin, crash a bit later like Apple II desktop or a2osX or have strange symptoms. Sometimes it's just one or two characters being replaced by the delete character other times it's weird issues like all visible text in AppleWorks just vanishing.
I'm thinking since some programs work, particularly older ones like Big Mac there might be an issue with extra 64K RAM or the 80 column firmware but I'm not sure and I'm a pretty big 8 bit noob outside of what I've seen in YouTube videos.
How might I best go about testing what might have failed? Since it's ROM version 255 I don't have the self test built in and trying to boot the Apple //c dealer diagnostics disk I found on the web just resulted in grey squares.
Anybody have a idea what might be the cause of this? Thanks.