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I think you should communicate with the former student loan letter via phone, rather than mailing letters to them because that appears it hasn't been working. It'd be a pretty simple conversation, you'd ask when they sold your loans, and after they confirm when they were sold you'd ask them why they are reporting late payments on your student loan after they sold it. Depending on how severe the late payments were, when they were, and how many late payments, it could potentially be a problem.
As far as getting the disputes removed you also need to contact the credit bureaus via phone, rather than mail letters. You should get your credit reports from each credit bureau at https://www.annualcreditreport.com/cra/index.jsp - each will list the report number which will allow you to call in and speak with a live person. Then when you get them on the phone you should follow the advice I give at http://ficoforums.myfico.com/t5/Mortgage-Loans/Best-Way-to-Remove-quot-Account-In-Dispute-quot-for-M... and the dispute remarks should be removed either during the phone call or within a few days. It's worked for everyone of my clients so far, so unless there are multiple disputes on the same trade line then it should work for you too.
I am not aware of any banks who are permitting less than 10% down on non-conforming jumbo loan amounts, which on a 1-unit property would be anything over $417k in the state of Wisconsin... so if you don't have 10% down it sounds like the lender you are working with could be your only option.