Try deleting all stopLoop calls and put it in the starSlider function
function startSlider(){
count = $("#slider > img").size();
stopLoop();
loop = setInterval(function(){
if (sliderNext>(count-1)) {
sliderNext = 0;
sliderPrev = 0;
};
$("#slider").animate({left:+(-sliderNext)*100+'%'},900);
sliderPrev = sliderNext;
sliderNext=sliderNext+1;
},6000)
}