The way you said it, do you mean it keeps getting hacked when you change it, or keeps getting changed constantly?
If so, you're being HACKED. Calm down, it happens.
I would advise connecting to SSH via putty. Run "w", and "netstat -nalp |grep "SHPORTHERE" to see whos connected using SSH.
There may be PHPShells, rootkits etc on your server. I would advise running 'cat /path/of/your/web/logs/* |grep "/x90/' to look for shell code.
Search for running perl scripts 'ps -aux | grep perl'.
I would also login to whm (if you can) and look at wheel group users, as well as viewing any resellers, and double checking no one else has root privileges (if they do, they can reset your root pass).
Also, are you using the root password elsewhere? For anything else at all? Perhaps a billing system .e.g. WHMAutoPilot etc?