[Tyhm]

If memory serves, all the nonlinear motion scripts he had came from me anyway, particularly the bird...

The trick is to modify this.xspeed and this.yspeed instead of x and y directly. So
if(dir=0) this.yspeed-=random(0.5,1); //try to go up - if you're going down, slow down some
//etc
while(abs(this.yspeed)>1) this.yspeed*=0.9; //maximum speed=1 tile per call
//etc
this.testy=y+this.yspeed;
//all the usual onwall: if it hits the wall, this.testy=y; this.testx=x;
y=this.testy;

You can get it from there easy. If you can't, give up, you're scripting out of your league.