Kari,
Language files are cached by default for performance reasons, so it is normal to see changes persists.
The surest way to clear the cache is reboot the server, though you can also do this in your web server as well. (There is a setting to turn this caching on or off in Language > Language settings > Cache language menu. The default is on and that's the best choice.)
Also, be aware that the edits you make do not change the quiz.php in the normal application Moodle lang/en_utf8 directory, but the custom strings you make are saved in a new directory created under your Moodle data directory called /lang/en_utf8_local. There you will find the quiz.php file with your language edits (and only your edits). This is cached too.
Once back to normal, try re-editing the quiz.php in Language editing to see what the original problem was. Some of these strings do contain syntax, punctuation, and variable names that must match exactly.
I suggest trying simple changes (eg. change one word) to the text that do not involve any variables and see if the problem recurrs. (It should not). Then add and test one change at a time to see if and when the issue arises again. From the description of the issue with student names in the gradebook, I would suspect one of the variables such as studentname or username may have an issue.
Randy