This is driving me nuts!

I'm getting some JSON from my server:

{"id262":{"done":null,"status":null,"verfall":null,"id":262,"bid":20044,"art":"owner","uid":"demo02","aktion":null,"termin_datum":null,"docid":null,"gruppenid":null,"news":"newsstring","datum":"11.06.2010","header":"headerstring","for_uid":"demo01"},

"id263":{"done":null,"status":"pending","verfall":null,"bid":20044,"id":263,"uid":"demo02","art":"foo","aktion":"dosomething","termin_datum":"11.06.2010","docid":null,"gruppenid":null,"datum":"11.06.2010","news":"newsstring","for_uid":"demo01","header":"headerstring"},

"id261":{"done":null,"status":null,"verfall":null,"id":261,"bid":20044,"art":"termin","uid":"demo02","aktion":null,"termin_datum":"25.06.2010","docid":null,"gruppenid":null,"news":"newsstring","datum":"11.06.2010","header":"headerstring","for_uid":null}}

This is how my JS looks like:

var user = 'demo02';

 new Ajax.Request('myscript.pl?someparameter=value', { method:'get', 
    onSuccess: function(transport){
        var db_json = transport.responseText.evalJSON(),
            propCount = 0,
            someArray1 = [],
            someArray2 = [],
            otherArray = [];

        //JSON DEBUG
        console.log('validated string:');
        console.log(transport.responseText.evalJSON(true));

        for(var prop in db_json) { 
        propCount++;
            if  ( (db_json[prop].art == 'foo') && (db_json[prop].for_uid == user) ) {
                someArray1.push(db_json[prop]);
            } else if( (db_json[prop].art == 'foo') && (db_json[prop].uid == user) ) {
                someArray2.push(db_json[prop]);
            } else if( db_json[prop].art == 'log' ) {
                otherArray.push(db_json[prop]);
            }
        }

        if(someArray1.length>0) {
            someArray1.map(function(el){
                $('someArray1target').innerHTML += el.done;
                //do more stuff
            });
        }

        if(someArray2.length>0) {
            someArray2.map(function(el){
                $('someArray2target').innerHTML += el.done;
                //do more stuff
            });
        }   

});

Sometimes, it works perfectly.

Sometimes, i get my JSON String (it appears in Firebug's "answer"-tab), but it won't log the JSON in console-log(). I'm not getting any errors and javascript is still working.

Next time after reloading, it might work, but it might not.

I cannot remotely imagine why this only happens sometimes!