js - converting array buffers to string

I'm getting some weird results when converting an array buffer to a string then displaying the output in a div.

I'm getting some GPS data from the USB port in a chrome packaged app. It converts the array buffer received from the port into a string and outputs. The functions are:

var onReceiveCallback = function(info) {    
    if (info.connectionId == connectionId && info.data) {
        $(".output").append(ab2str(info.data));
    }   
};

/* Interprets an ArrayBuffer as UTF-8 encoded string data. */
var ab2str = function(buf) {
    var bufView = new Uint8Array(buf);
    var encodedString = String.fromCharCode.apply(null, bufView);
    return decodeURIComponent(escape(encodedString));
};

I have a start and stop button to obviously start and stop the reading of data from the gps device. When I start it the first time it works and outputs as expected, something like:

$GPGGA,214948.209,,,,,0,0,,,M,,M,,*41 $GPGSA,A,1,,,,,,,,,,,,,,,*1E $GPGSV,1,1,01,07,,,33*7F $GPRMC,214948.209,V,,,,,0.00,0.00,270814,,,N*4C $GPGGA,214949.209,,,,,0,0,,,M,,M,,*40 $GPGSA,A,1,,,,,,,,,,,,,,,*1E $GPGSV,1,1,01,07,,,34*78 $GPRMC,214949.209,V,,,,,0.00,0.00,270814,,,N*4D

but then when I stop it, and restart it, although I clear the output div, the output data seems to be mixing in with the previous result. Like:

$$GPGPGGGGAA,,221155115544..202099,,,,,,,,,0,0,0,0,,,,,,MM,,,,MM,,,,**4455 $$GGPPGGSSAA,,AA,,11,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,**11EE $$GGPGPGSSVV,,11,,11,,0022,,0077,,,,,,3344,1,177,,,,,,3311**77FF $$GGPPRRMMCC,,212155115544..220099,,VV,,,,,,,,,,00..0000,,00..0000,,227700881144,,,,,,NN*4*488 $$GPGGPGGGAA,,221155115555..220099,,,,,,,,,,00,,00,,,,,,MM,,,,MM,,,,**4444 $$GGPPGGSSAA,,AA,,11,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,**11EE $G$GPPGGSSVV,,11,,11,,0022,,0077,,,,,,331,1,1177,,,,,,2255**77FF $$GGPPRRMMCC,2,21155115555..220099,,VV,,,,,,,,,,00..0000,,00..0000,,227700881144,,,,,,N*N*4499

Its like a buffer or variable isnt being emptied, or something else crazy that I cant figure out. Any pointers appreciated.