It sounds as if you have been very patient with this process. One never wants to lose a truly motivated seller but extending the closing as many times as you have is generally a good indicator of significant problems. Have you signed the most recent contract extention?
I would consider telling this buyer that you want to sell the property to them but can't afford to keep it off the market any longer and will be moving toward putting it back on the market. In the event they do resolve their credit issues, you will be more than happy to resume their purchase process, honoring the previously agreed to terms. However, in the event you find another motivated buyer you will be moving toward closing.
It may be time to protect your interests by moving forward but not closing the door....