SBG6580 Password Issues

I have an Arris SBG6580 that won't accept the recorded username and password (recorded on paper and in Chrome). I did the router default and was able to login with the default user name and password (admin/motorola). This brings up a text box that requires me to change my user name and password before proceeding - this is not the behavior noted in the user's manual. Once I have entered the new user name and password in the text box (along with a confirmation of the old password, motorola), the unit directs me to log in again. However, when I attempt to log in, I get an "invalid user name / password" message. I can then login again using the default user name and password, but I get the same change text box again. If I attempt to change the user name and password again, I get an error message that the old password confirmation (motorola) was not entered correctly. I tried the new password in this box, which did not help. The only way to log in again is to re-default the router, at which point I am back to where I was before but I still cannot change the user name and password. The router is working, it reaches the internet OK, but I cannot make any of the port allowance I need to run several features and servers we have. Other than replacing the unit, is there anyway to get this thing to behave the way it should?