Clearing the cache changes the nice menu every time Maybe this isn't a cache problem, but I am using Drupal 6, I have cache disable and I am editing a custom nice menu CSS file.  

I didn't see changes right away to my nice menus, so I cleared the cache in the performance page, and the menu would revert back to old changes. When I cleared the cache again, it randomly found new settings for Nice Menu.
I then installed Devel, and cleared the cache, but it didn't help. Clearing the browser cache didn't help either.

Do you have any idea?
If you have turned CSS file aggregation off, changes take effect immediately unless you have an external cache like Varnish of the css file. If you are not seeing changes after checking this, maybe your CSS file is not included or the rules implemented is overridden by other CSS rules.