Flashing when the next screen is updating

Hi, I'm toying with something at the moment. I've got a scrolling background, and I'm trying to change the background on the next screen. It's making the changes, and is changing them at the right times, but I get a quick flash on the screen every time it runs.

I've attached the files as well as the rom. I haven't finished setting the background attributes, so please ignore that for the time being. I'm using NESASM and running it on MESEN.

Here is the code:

Code:

  .inesprg 1    .ineschr 1     .inesmap 0    .inesmir 1    ;;;;;;;;;;;;;;;  .bank 0  .org $C000   .rsset $0000bgLowByte     .rs 1bgHighByte    .rs 1scroll        .rs 1ppuctrl       .rs 1currentScreen .rs 1  RESET:  SEI           CLD            LDX #$40  STX $4017     LDX #$FF  TXS            INX            STX $2000     STX $2001     STX $4010   vblankwait1:       BIT $2002  BPL vblankwait1  clrmem:  LDA #$00  STA $0000, x    STA $0100, x  STA $0400, x  STA $0500, x  STA $0600, x  STA $0700, x  LDA #$FE  STA $0300, x  INX  BNE clrmem   vblankwait2:     BIT $2002  BPL vblankwait2    lda #%00000000  sta $2000  lda #%00000000  sta $2001  loadPalette:    lda #$3f  sta $2006  lda #$00  sta $2006   ;first palette  lda #$0f    sta $2007  lda #$11  sta $2007  lda #$10  sta $2007  lda #$30  sta $2007  ;second  lda #$0f  sta $2007  lda #$06  sta $2007  lda #$16  sta $2007  lda $26  sta $2007setPPUControl:  lda #%10010000  sta ppuctrl  initScroll:  lda #$01  sta scroll  setCurrentScreen:  lda #$00  sta currentScreen  drawBackground1Tiles:  ldy #$00  ldx #$00  lda $2002  lda #$20  sta $2006  lda #$00  sta $2006    lda #LOW(firstBG)  sta bgLowByte  lda #HIGH(firstBG)  sta bgHighByte    bgLoop1:  lda [bgLowByte], y  sta $2007  iny  cpy #$00  bne bgLoop1    inc bgHighByte  inx  cpx #$04  bne bgLoop1  drawBackground2Tiles:  ldy #$00  ldx #$00  lda $2002  lda #$24  sta $2006  lda #$00  sta $2006    lda #LOW(secondBG)  sta bgLowByte  lda #HIGH(secondBG)  sta bgHighByte    bgLoop2:  lda [bgLowByte], y  sta $2007  iny  cpy #$00  bne bgLoop2    inc bgHighByte  inx  cpx #$04  bne bgLoop2loadAttributes:  lda $2002  lda #$23  sta $2006  lda #$C0  sta $2006    ldx #$00attrLoop:  lda bgAttr, x  sta $2007  inx  cpx #$40  bne attrLoop      resetPPU:  lda ppuctrl  sta $2000  lda #%00011110    sta $2001  Foreverloop:  JMP Foreverloop    NMI:  lda #$00  sta $2003  lda #$02  sta $4014    inc scroll  lda scroll  cmp #$FF  bne setScroll         inc currentScreen  ;jsr updateScreenByPage   jsr updateBaseNametable   RTI setScroll:  lda scroll  sta $2005  lda #$00  sta $2005   rts  updateBaseNametable:    lda #%00000000  sta $2000    jsr updateScreenByPage     lda ppuctrl  eor #%00000001  sta ppuctrl  sta $2000      rts  updateScreenByPage:  lda currentScreen  cmp #$03  beq setUpScreen3    cmp #$05  beq setUpScreen4  rts setUpScreen3:  lda $2002  lda #$23  sta $2006  lda #$33  sta $2006  lda #$02  sta $2007  rts setUpScreen4:    lda $2002  lda #$23  sta $2006  lda #$33  sta $2006  lda #$00  sta $2007    lda #$23  sta $2006  lda #$0C  sta $2006  lda #$02  sta $2007    lda #$23  sta $2006  lda #$2C  sta $2006  lda #$02  sta $2007  rts ;;;;;;;;;;;;;;  .bank 1  .org $E000  firstBG:  .include "flight1.asm"secondBG:  .include "flight2.asm"bgAttr:  .include "bg_attr.asm"    .org $FFFA      .dw NMI                          .dw RESET                        .dw 0          ;;;;;;;;;;;;;;    .bank 2  .org $0000  .incbin "flight.chr"