Problema de contenido de cambio de tamaño de Scrollview

I have an issue with scroll view. I have implemnted two font resize buttons to increase or decrease text font size for the text view embedded in the scroll view. But after decreasing font size, I calculate the scroll view content and reize it to adapt to the new text view height. But every time I modify text font, it scrolls down for every step.

Aquí está el código:

- (IBAction)decreaseText:(id)sender {

    int MyTextSizeMin = 8;
    int MyTextSizeMax = 20;

    // disable buttons when they're out of the range.
    BOOL smallerEnabled = textFontSize > MyTextSizeMin;
    BOOL biggerEnabled = textFontSize < MyTextSizeMax;
    [self.decreaseText setEnabled:smallerEnabled];
    [self.increaseText setEnabled:biggerEnabled];


    height =0;

    CGFloat secondLabelsize = texto.font.pointSize;

    [texto setFont:[UIFont systemFontOfSize:(secondLabelsize-1)]];

    CGRect frame = texto.frame;

    frame.size.height = texto.contentSize.height;

    self.texto.frame = frame;

    height +=texto.contentSize.height;

    height += self.imagen.image.size.height;

    height += self.fecha.frame.size.height;

    height += self.titulo.frame.size.height;

    height += 200;

    self.scrollView.contentSize=CGSizeMake(300.0,height);

    [self.scrollView setNeedsDisplay];


}

How can I disable this undesired scroll?

Muchas gracias!