Well... you are about 3/4 of the way there... you already have your JSON as text.
The problem is that you appear to be handling this string as if it was already a JavaScript object with properties relating to the fields that were transmitted.
It isn't... its just a string.
Queries like "content = data[x].Id;" are bound to fail because JavaScript is not finding these properties attached to the string that it is looking at... again, its JUST a string.
You should be able to simply parse the data as JSON through... yup... the parse method of the JSON object.
myResult = JSON.parse(request.responseText);
Now myResult is a javascript object containing the properties that were transmitted through AJAX.
That should allow you to handle it the way you appear to be trying to.
Looks like JSON.parse was added when ECMA5 was added, so anything fairly modern should be able to handle this natively... if you have to handle fossils, you could also try external libraries to handle this, such as jQuery or JSON2.
For the record, this was already answered by Andy E for someone else HERE.
edit - Saw the request for 'official or credible sources', and probably one of the coders that I find the most credible would be John Resig ~ ECMA5 JSON ~ i would have linked to the actual ECMA5 spec regarding native JSON support, but I would rather refer someone to a master like Resig than a dry specification.