flip error image

it happens how need flip image on screen and after install service pack2 :) bad black flash

My system:

Microsoft windows xp professional version 2002
Service Pack 2 Pentium 3 500 MHz 320 MB ram
Graphic card: NVIDIA RIVA TNT2/TNT2 PRO (Microsoft corporation)

test code:

Global screenw = 800:screenh = 600
Graphics screenw,screenh
Global imno = 1000 ; play with this number
SetBuffer FrontBuffer()
Text 400,300,"Please wait grabbing images......",1,1
Global temp = CreateImage(64,64,imno)

Type Block
Field imageno
Field x#,y#
Field dir
End Type

Global b.Block

SetBuffer BackBuffer()
getimage()

draw()
Flip

; main loop
While Not KeyDown(1)

move()

Flip
Cls
Wend

Function move()

For b.Block = Each Block
DrawImage temp,b\x,b\y,b\imageno
b\x = b\x +Rnd(-4,4)
b\y = b\y +Rnd(-4,4)
If b\x <= 0 Then b\x =1
If b\x >= 800-64 Then b\x =b\x -1
If b\y <= 0 Then b\y =1
If b\y >= 600-64 Then b\y =b\y-1
Next
Text 400,0," Images On Screen:" + imno

End Function


; draw the images at random positions around the screen
Function draw()
SeedRnd MilliSecs()

For b.Block = Each Block
b\x = Rnd(0,700)
b\y = Rnd(0,500)
DrawImage temp,b\x,b\y,b\imageno
Next
End Function

; Draw and grab the image
Function getimage()
Color 255,255,192
Oval 32,32,32,32
Color 130,133,248
Oval 32,32,31,31
Color 255,255,0
Oval 39,39,4,4
Oval 52,39,4,4


For im = 0 To imno-1
b.Block = New Block
GrabImage temp,0,0,im
b\imageno = im
Next

Cls
End Function