My swf plays only twice and stops.

1) Change the code you have in frame 1 to:

stop();

var nextF = 2;
var prevF = 2;

larrow.addEventListener(MouseEvent.CLICK, fl_ClickToGoToAndPlayFromFrame_5);

function fl_ClickToGoToAndPlayFromFrame_5(event:MouseEvent):void
{
     gotoAndPlay(prevF);
}

rarrow.addEventListener(MouseEvent.CLICK, fl_ClickToGoToAndPlayFromFrame_6);

function fl_ClickToGoToAndPlayFromFrame_6(event:MouseEvent):void
{
     gotoAndPlay(nextF);
}

2) Change your code in frame 2 to:

stop();

nextF = 1;

prevF = 1;

3) Change your layer with the arrow control so that it only has the one keyframe in frame 1 so that the same button instances are used along the entire timeline.  The easiest way to do this is to right click frame two of that layer and choose Clear Keyframe.

If you need to add more frames, just use the code in frame 2 and adjust the values of prevF and nextF to suit where you want things to go.

The problem with what you had before is that you end up assigning both listeners for the two frames to the buttons, so when you click one after both have been assigned, both functions get executed.  With the changes above you only ever have one function for each button.

You'll better your chances of getting help if you don't ask people to download files and instead explain your design and show the code that is relevant to the problem.

What is the code your file is using for the buttons and how is it set up in the file?