Do not, under any circumstances, contact a collection agency. It's a great way to have whatever it is stick to your credit report.
In a situation like this, I think that
CreditBoards.com - Credit Help, Credit Repair Tips, News, Forums is your best bet. It specializes on things like this. You'll probably need to remove old addresses from your credit reports, order copies of any credit report affected, and write letters (the board provides the text) asking the credit bureau (Transunion, and all) to verify the item in question. The credit bureau will contact the collection agency for proof of the item and if it's not legitimate it will be removed and it's all taken care of.
I helped my fiance go through this about a year ago. Not a fun process, but in her case the item was not legitimate (medical billing error) and we got it taken care of.