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Since you have already sent certified letters the next step is to put rent is escrow. In Ohio I can go to the clerk of courts and give them my rent check. The landlord can not receive the check until the issue is settled, yet he can not evict for failing to pay rent. If the problems are still not fixed then you can request the money from escrow to pay for the repairs yourself.
The exact process should be similar in every state but there are requirements to follow. Search for a legal aid society in your city and you should be able to get free advise from a local lawyer that would know exactly what to do.