Talked with Neil yesterday.

He had a simple solution to this.

From Grub it is possible to edit VESA mode with higher refresh. Like this.

VESA=1024x768@100

This will give you 100hz display if your monitor supports it.
Here it is of cause possible to experiment with different resolutions and refresh rates. At least it works :)

edit: This does as I asumed solve the issue with Janus-UAE. Now it is very smooth. As you know 50hz * 2 = 100hz

I just got a CRT montior 21"
I see different refresh rates selecting different screenmodes but all are 60hz even if I for example select 75hz.
This is using VGA connection from my laptop. IntelGMA.

60hz is all good for LCD screens but for CRT it is flickering.

What I also found in our Amiga 68k emulator, Janus-UAE is that
sometimes updating the screen running games or demos hangs a little. It is simply not updating the picture fast enough it seams. This is both with LCD and CRT. I'm not sure why this happen but with Windows and WinUAE it is not like that.
Does it have anything to do with some convertion. 50hz PAL to 60hz NTSC or is it something else?
Vsynch?

Could someone create a 640x480 100hz screenmode for use with Amiga 68k emulation?

When nothing saved in screenmode.prefs it shows the refresh rates but after one screenmode is saved it does not show up again unless you delete screenmode.prefs and start it again.
Another question is if it would be possible to select different refresh rates for different screenmodes?
Lets say I have wanderer (workbench) in 85hz and want to run Janus-UAE in fullscreen 640x480 100hz?