Is ACS the same as AES? If not go there they will help you out...they took over SLSC...
AES
Here is a hint..even though it is their resposibility to deal with the address change you should always cover your ass..next time a bill does not come contact them and handle it the first missed month...Student Loans are big default loans so if you are willing to work with them and get in their face they will work with you...Student loans do suck I have been paying mine for 10 years and I still have $4,000 left