Solution:
There are two things you should take care of:
1) detecting wether slider is visible for user. There are several solutions for this, for example this or this
2) stopping/starting slider depending on slider visibility. Combining it all together, the code would look like this. It is conceptual and untested, but the idea is clear, I think.
jQuery(window).scroll(function($) {
function isScrolledIntoView(elem)
{
var docViewTop = $(window).scrollTop();
var docViewBottom = docViewTop + $(window).height();
var elemTop = $(elem).offset().top;
var elemBottom = elemTop + $(elem).height();
return ((elemBottom <= docViewBottom) && (elemTop >= docViewTop));
}
if (isScrolledIntoView($('.rd_testimonials'))){
$('.rd_testimonials').carouFredSel({auto: false});
} else {
$('.rd_testimonials').carouFredSel({auto: true});
}
});