I'm sure the cable worked fine in other monitors, but is the brand and model of the cable in this list?
https://www.displayport.org/product-category/cables-adaptors/
Yes, the cable is in the list.
Let me ask you this, a question of logic.
If the monitor works, at 144hz, with freesync off, and does not flicker, but it does when you turn freesync on, why would you look at the cable to be the issue?
Bandwidth is bandwidth, enabling freesync does not increase bandwidth, that is dependent on the resolution, color depth and frequency / framerate.
The amount of data / bandwidth passing through the cable is the same at a given resolution, color depth and frequency, regardless of whether freesync is on or off.
At 3440x1440, 144 Hz, 8 bit color we are talking about 21.40 Gbps which is less than half the capacity of an 8K 60Hz / 4K 120Hz Vesa certified cable which should work at 48Gbps.
The fact that ALL of these 3440x1440, 100 - 144Hz curved VA panel $300 - $600 monitors flicker and none are officially G-Sync compatible certified should tell you something.
I am currently using a monoprice monitor with the same specs, it actually has in my eyes a better quality panel, less smearing, no dead pixels, clearer text and cost less, than the IIyama, however... there is a high pitch 9.7KHz tone coming from the left of the panel where the power input is, seems to be the backlight driver / power supply, I'm not a monitor manufacturer, I can't say what it is for sure but it's only audible when the panel is lit so...
The sound drives me crazy and I am on my second panel, both displays are flawless except for the high pitch noise which I can't stand, leaves me with my ears wringing the whole day. Oh, and it flickers, however, less than that IIyama did.
So if anyone is slightly tone deaf or can't hear above 9KHz give it a try, to me the panel is superior (OSD is horrible but who plays around with that after you set it up good once). The monoprice Dark Matter 34" 144Hz, 1500R curved display.
Hey, maybe I was just super unlucky and got two ones with a bad power unit that squeals in a row.
I will be sending the second unit back as well.
I have another IIyama ordered from Amazon for delivery in February, will see if I just had a super bad unit the first time round, hey, maybe I can snag a 30x0 Nvidia GPU or a 6x00 AMD and try my luck with that, I currently have a 2060s.
If that IIyama also flickers a lot or has other quality issues then I will just stop with 34" ultrawides for a while (I can't afford the €900 LG), I never should have gotten rid of my 16:9 165Hz 2K G-Sync compatible certified Dell, it was so beautiful, no smearing, no flickering, alas I wanted the immersion that this 34" ultrawide offered however the quality / performance issues are hard to overlook.