Hi, I had the same setup and flickering and solved it this way:
Signal from Source (PC) is 60Hz. HDMI 2 AV converter is in NTSC mode. There is a little slider next to the power input. (NTSC is 59.94Hz). And set the value of the grabber property "standard" to "NTSC".
So it might be the same for you that the input is 60Hz (likely if you are in the EU).
Before the slider was on "PAL". Which works with 50Hz and can cause this kind of flickering due to different frequencies. Imagine it like someone talking too fast and you can understand most but not all of the words.
Changing the baudrate, core_freq or other frequencies changed the behavior of the 'random' flickering. But did not lead to a solution for me.
Now there is only a little flickering left in extreme situation like: left half of the screen black(#000000) and right half of the screen dark grey(#111111). Only the LED's right on the edge between these two colors are flickering a little bit. But that is due to the fact, that the converter does not provide a static output for a static input. Therefore true black input does not imply true black output. This flickering gets a little bit reduced by enabling the smoothing feature.