1998 Identity theft

In 1998 I started receiving calls from collection agency. At first I just blew it off that they had the wrong person because I pay my bills and had never ever had a collection agency call me. After trying to explain to them it wasn't me I finally contacted an attorney who wrote to them and also I contacted the credit bureau. Thought everything had been taken care of. In 2000 once again another collection agency started calling.. Same account from 1998. I found out the first collection agency sold to 2nd collection agency. So my attorney sent information to them. Everything was fine till now. 3rd collection just sent me collection letter with same account as first collection agency. Must of been sold to 3rd collection agency. My thought is why didn't the first collection agency do something with this account? Do I have to do something everytime a collection agency decides to sell accounts. I feel like this night mare will never end. Any suggestions?

Ronda