Controlling circular movement help

HI there
I been trying to
controll a mc that is rotating using this actionscript
onClipEvent (load) {
radius=100;
xcenter=250;
ycenter=200;
angle=0;
speed = 5;
}
onClipEvent (enterFrame) {
_x = Math.cos(angle*Math.PI/180)radius+xcenter;
_y = Math.sin(angle
Math.PI/180)*radius+ycenter;
angle += speed;
if (angle>359) {
angle -= 360;
}

}

What I am trying to do is be able to stop it, and depents of the button that I click this will stop at 45 grades or any grade that I especific,
and How can I play this backwards???

If any one know sothing or can guide me I will be thank full

sorry for my english, I am not at native English speaker.

thank you.

Alejandro