Ok, what a monitor you used? I'm using C1084 and Torsten adjusted VSync on C1901. If you used any converter, don't know - but I used also RGBI2S-Video and there's same flickering. If you don't believe that it can work - try it at least in Z64K.
Commodore 64 was great, Commodore 128 is bigger, better, faster and more powerful... Commodore 65 was almost here, but C256 is coming and it will be earthquake...
I tried on my 15khz capable BenQ VGA LCD, it didn't work. I also tried on 2 VGA monitors with the scandoubler attached to get 31khz, It still didn't work. I don't have a 15khz CRT available to me. I guess I could try it in a SCART TV but as I won't usually be using that I didn't bother. I may try it on a CRT VGA monitor if I get bored...
And have news: I got it partially display by my Amberry CGA to VGA converter.
It displays not whole height, but I found new mysterious result - attributes looking like working in 100x50 mode.
Look at screenshots from converter below:
While the horizontal display can be greater than 82, one thing to take note of from what I've observed is you will start getting repeating characters on each line. See VDC horizontal display greater than 82.
I created else one test: characters 33-127 and colors 1-16
Look below what I understand not.
I added some lines in your image you sent me via email which I hope helps you understand. I assume it's the same as the one you posted here.
Basically the characters between the first set of lines matches the characters between the second set of lines. The behaviour is all discussed in the VDC horizontal display greater than 82 link I provided above.