There are many reasons for monochrome LCD screen flashing, but the frequency is not too high. Otherwise, the high refresh rate of TV will become a joke? The human eye basically has no flicker sense for refreshing the image above 60Hz. The LCD industry is basically above 50Hz, and the high one is 120Hz. The higher the higher, the less flicker, but the power consumption is large.
If you can confirm that the frequency of the screen itself is high enough (software setting), you can confirm whether it flashes differently under different light sources. If your monochrome LCD screen has a backlight, you'd better turn off the backlight and see if it flashes in the sunlight to confirm whether the backlight flashes. In the same way, compare the effect of artificial light source (fluorescent lamp) and the sun, because your screen may flicker at a frequency similar to that of artificial light source at some specific frequencies.
If it is the problem of the monochrome screen itself, the software setting shall be mainly considered to improve the frequency related parameters. If it cannot be adjusted, the OSC frequency of the IC shall be increased to see if the flicker has changed. If your screen is not cog and has separate row and column driver chips, it mainly depends on the chip settings of the row (common). You can measure the relevant signals to see if the row frequency is as set by the software.