The screen flicker is an artifact of the constant access of the 6502 to the video RAM. The raster gets blanked momentarily during R/W multiple times per scan line. The program is constantly polling the keyboard & writing to the screen causing the flicker. It's annoying, but operating as expected.
Based on the screen image, the access to RAM at $00xx and $01xx is corrupt although we don't know how (stuck bit or write fail etc.)
In addition data line D3 is stuck high on video RAM since all characters have binary 8 value bit set. However reading D3 works from the ROM so it is not stuck high on the CPU side -- the program is running. So it's possible it has something to do with pin 12 of U25 (8T28) not being connected to pin 14 and the D3 line from the 6502, or pin 13 is somehow wired high on the video RAM data bus. According to the schematics, pin13 U25 (8T28) is wired to to pin 11 U39 (RAM) and pin 2 of U41 (CHARGEN).
On the
schematics I have, U40 (2114) is used for D4 to D7.