I can see why you used a preloaded CHD hard disk. For years people recomended ct486 to run PC games, I've been always using at486. I tried to boot ct486 with msdos5_35, msdos6, msdos62u and msdos62a from the softlist, however it hangs forever before the blue setup screen appears. That's not a problem with at486 and it seems the issue goes back from at least 0.257 (last time before today I upgraded MAME). I'll see if I can find the regression and fill a bug report in case there isn't any or someone else hasn't done it.