Why are there waves in my rectangle?

Graphical Glitches? Is there a way to fix the graphical waves thingy unsmooth movement... if you run my example and u move that rectangle ull see waves on the sides when moving randomly appearing... how do u fix this? In alot of engine this appears for some reason.

Graphics 800,600,0

Global GameExit:Int=False

Local P1:TPlayer=New TPlayer; P1.Create(10,10)
While Not GameExit=True
	If(KeyHit(KEY_ESCAPE))Or(AppTerminate())Then GameExit=True
	
	P1.Controles()
	P1.Drawing()
	Flip; Cls
Wend
End

Type TPlayer
	Field Health:Int, Mana:Int
	Field xpos:Int, ypos:Int
	Method Create( xstart:Int, ystart:Int)
		xpos=xstart
		ypos=ystart
	End Method
	Method Controles()
		If KeyDown(KEY_UP) 		Then ypos :- 3
		If KeyDown(KEY_DOWN) 	Then ypos :+ 3
		If KeyDown(KEY_LEFT) 	Then xpos :- 3
		If KeyDown(KEY_RIGHT) 	Then xpos :+ 3
	End Method
	Method Drawing()
		DrawRect xpos,ypos,70,120
	End Method
End Type