Unexpected token on jsonp request

[Toni Thomä]

hello, 
i´m firing an ajax/jsonp reqeust to my cloudant app: 

        var obj = $.ajax({
            url: "http://xyz";,
            dataType: 'jsonp',
            success: function(data) {  
                //SOME CODE  
            },
            error: function() {
                //SOME CODE
            }
        });

most of the time the response is ok and i can read out my data. 
but sometimes I´m getting the following error (with the same query string that 
already worked): 

        Resource interpreted as Script but transferred with MIME type 
text/plain.
        Uncaught SyntaxError: Unexpected token :

i need to make this request an jsonp request, AFAIK jsonp returns as a script 
and gets executed by the browser. 
do i have to set a request header? 
(I´m using GoogleChrome)

cheers, 
toni