flashing screen?

"Flip False" copies the buffer to the screen without waiting for a VSync. The videocard constantly copies the buffer to the screen and with "Flip False" it could be halfway the process. That will cause the vertical interference.

I found out the best way to test this is to make the screen alternate between black and white:

Graphics 800, 600, 0, 1

;test this program by adjusting these constants
Const flipenable 	= False
Const vwaitenable 	= False

.r

	i = i + 1
	ClsColor 0,0, $FFFFFF * (i Mod 2)
	Cls
	If vwaitenable Then VWait 
	Flip flipenable

	;esc=exit
	If KeyHit(1) Then End
	
Goto r

I have seen people using VWait before calling Flip false, which accordingly to the Blitz Help should act differently.