Is this a bug with flash or just my variables?

For some reason my first if statement is allways fulfilled, even when I set the varaiable to different values, I cant work out why. My variables are not used anywhere else in my site. I have spent hours and hours and still cant work out what is wrong. I desperatly need someones help, time is running out.

_global.roller=0
if (_global.roller=5){
var speed = 0;
}

else {
var speed = .5;
if (_root._xmouse >186 && _root._xmouse <624){

if (_root._ymouse < 6 || _root._ymouse > 374) {
speed = speed * 0.950000;
} else {
speed = -_ymouse / 90;
}}
}

this._visible = true;