This is beyond ridiculous

So my issues aren't to do with anything complicated I've done, it seems. They are to do only with this line:

this.y -= (speed - pSpeed);

This seems to be because I am using -=, since I have run some simple tests which work under some circumstances and get a white screen of DOOOOM under most others.

This works:
this.y = 300 + whiteLineOffset;  //whiteLineOffset is a variable used elsewhere that changes over time.

These do not work:
this.y = this.y + 1;

this.y += 1;

var temp:Number = this.y;
this.y = temp + 1;

I am stumped.