Here's the recording, SHIFT+C causes this pattern, other single line horizontal chars cause no such flicker:
https://youtu.be/gyQ4ZhE1fEg
https://youtu.be/gyQ4ZhE1fEg
So this means there shouldn't be any snow then... so the "I/O chip" (the VIA I guess) could perhaps not be functioning properly, or something else in that circuit?
That "Fire" game has a small ML routine at 826 IIRC, when I just ran the SYS command for that I got the glitches.
.C:033a A0 FF LDY #$FF
.C:033c 84 01 STY $01
.C:033e A0 84 LDY #$84
.C:0340 84 02 STY $02
.C:0342 A0 00 LDY #$00
.C:0344 A9 00 LDA #$00
.C:0346 A2 00 LDX #$00
.C:0348 B1 01 LDA ($01),Y
.C:034a C9 DF CMP #$DF
.C:034c F0 03 BEQ $0351
.C:034e 4C 58 03 JMP $0358
.C:0351 A9 E9 LDA #$E9
.C:0353 91 01 STA ($01),Y
.C:0355 4C 7F 03 JMP $037F
.C:0358 C9 E9 CMP #$E9
.C:035a F0 03 BEQ $035F
.C:035c 4C 66 03 JMP $0366
.C:035f A9 DF LDA #$DF
.C:0361 91 01 STA ($01),Y
.C:0363 4C 7F 03 JMP $037F
.C:0366 C9 CD CMP #$CD
.C:0368 F0 03 BEQ $036D
.C:036a 4C 74 03 JMP $0374
.C:036d A9 CE LDA #$CE
.C:036f 91 01 STA ($01),Y
.C:0371 4C 7F 03 JMP $037F
.C:0374 C9 CE CMP #$CE
.C:0376 F0 03 BEQ $037B
.C:0378 4C 7F 03 JMP $037F
.C:037b A9 CD LDA #$CD
.C:037d 91 01 STA ($01),Y
.C:037f C6 01 DEC $01
.C:0381 F0 03 BEQ $0386
.C:0383 4C 48 03 JMP $0348
.C:0386 C6 02 DEC $02
.C:0388 A9 FF LDA #$FF
.C:038a 85 01 STA $01
.C:038c E8 INX
.C:038d E0 04 CPX #$04
.C:038f F0 03 BEQ $0394
.C:0391 4C 48 03 JMP $0348
.C:0394 60 RTS
Don't think that routine is the killer POKE, I tried that poke with "Fire", there was snow all the time and game was a bit faster.
E84B ACR Aux. control register; set to $00 at power on
7-6 timer 1 control
5 timer 2 control
4-2 shift register control
1 port B latch
0 port A latch
E84C PCR Peripheral Control Register; set to $0C or $0E at power on
7-5 CB2 control (user port pin M) (note 3)
4 CB1 control (note 3)
3-1 CA2 control (graphics mode) (note 3)
0 CA1 control (note 3)
You'll find quite a few games that have screen garbage on the 2001 versus the newer models.