Consult an attorney.
Continue to make payments on the product until the case is resolved, unless your attorney advises you to do otherwise.
Keep detailed records of problems and receipts of attempts to repair products you suspect may be defective. Include the names of persons you talk with whom you may want to call to testify as expert witnesses on your behalf.
Some suits may be pursued in Small Claims Court if the sum is less than $3000.
If it is necessary for you to retain an attorney, you may be able to recover his/her fees as part of your suit.