Sorry for this digging up, please tell me if the forum rules ask for a new topic instead of using a 4 years old one.
Nevertheless, all the above information helped me a lot in order to find a solution for a stable Dreamcast image on my CTM 644 Monitor. I simply cut the pin 10 of the Dreamcast AV Connector and connected a Sync Stryper to it. The LM1881 of the Sync Stryper is powered through the pin 5 of the AV Connector, and ground goes through pin 1.

I use an official (I guess) Dreamcast RGB cable which only has pins 1, 2, 3, 4, 7,
10, 14, 15, 16 available. The idea was to use pin 10 for a perfect Sync signal.
The result is great compared to previously. No more diagonal screening however the image is still very dark and bluish

The defaults certainly come from the cable. Used with a CPC 1628 or a MegaDrive the colors of the monitor look great. However, on a CRT TV or a flat screen TV, the colors of the Dreamcast are totally normal...
My next try will be to disconnect all the capacitors used in the Dreamcast cable but if you have other suggestions in order to get a non-bluish and a lighter image please let me know.

My cable is slightly different from the picture above with Composit coming from pin 10 of the AV Connector and a capacitor right befor pin 8 of the SCART.
Here is a little video of the curent situation. Thank you in advance for your advices.
https://youtu.be/HJ3UaXbbbDY[Edit]The solution was to connect the ground
If you connect the shielding of your
Scart --> Amstrad cable on pin 5 of the female DIN and on pin 21 of the Scart you get a perfect image:
TL;DRTo cancel the diagonal scrolling: connect a sync Stryper to the Composite output of the DC
To get a perfect picture: don't forget to connect the shielding of your cable to the ground