Date of last acitivity sold and redated more twice in collections

The company (sun dial) that I bought the clock from went out of business and sold all of their accounts to another company. For some reason I was not informed and did not know where to pay my payment. In February of 2000 another company (the associates) contacted me on the phone about the account. I was told that the account was bought by them when the original closed. I paid the account and they sent me a letter of payment in full. Several months later I receive another call from another company(city financial) saying I owe this money. I told them I paid the account. They verified that the account was bought from the associates because the associates, also know as ACB recovery, had gone bankrupt. City financial also told me that it had not been paid. Since then the account has been sold to different companies each time giving a new report date and adding some finance charges.
I have disputed this several times and have gotten nowhere.
*How can I get this off my credit?
*Is there a way to get the history of this one account?
*Hasn't the statute of limitations run out?
*Can they really legally change the reporting date? (the most recent was 3/2007)
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Stop the silliness. Have them legally vaildate the debt.
Send confirmation delivery mail and keep copies of what you sent.