4th screen flickering

[Axlkilla]

HEllo

I have a TH2GO connected to 3 Iyama Prolite 2403WS for a 3X1680x1050 @59Hz. Everything runs fine, but i recently wanted to add my old monitor Samsung Syncmaster T220. I plug it onsecond DVI from my GTX 470 (first output used by Matrox TH2GO) I start computer, the Samsung is well recognized at 1680x1050 @60Hz, and the desktop is extended, but it starts flickering like the screen gets on then off then on, then off...
I just can't simply use this.
The DVI cable is 10 meters long. I'v been todl that it should be ok, but somebody could confirm that?
If the cable could be a problem, is there also other things that could be the reason? I want to be sure i cannot fix it in another way than using shorter cables.

Thank you for answers.

CU

Axl