Looks like a version of the Vigenere Tableau, except that was broken centuries ago.
The Vigenere Cipher can be broken (if you know the length of the key) using frequency analysis. The Tableau in this case looks like a variant of the Caesarian Cipher, but that does not affect the method used to break the code.
So it seems to be a simple Vigenere cipher. All we need to do is find the length of the key, which must be done with trial and error. Although I'd be inclined to try 'kryptos' as the key initially...
Looking at the parts thus far deciphered, it seems I wasn't too far off. The problem with breaking codes such as this is that there are so many ways of using a matrix of letters...
