TH2G digital edition problem?

Hi
I am hoping somebody has expereined this before:
I have a macpro hooked up to a TH2G, at 3x1280x1024 on output 2 and another monitor at 1920x1080 on output 1. All works when I have local monitors attached.
the moment i split the signal from the TH2G things stop working. I can get a signal output one from the TH2G once I go thru a DVI splitter or DVI fibre extended which has a local monitor loop thru.
Nothing seems to read output 2+3 from TH2G apart from my local monitors. I suspect some EIED problem, but what?
Many thanks,
Sven

BTW: I am running 10.5.8 on DuoCore 2,66 and on X1900 card

You're exceeding the video card's capability. When you are using the TH2G on one output, don't exceed 1280x1024 on the second.

1920x1080 and 3840x1024 don't play nice

Mmmmh.
But how come I can have 1920x1080 and 3840x1024 working fine when no external devices are attached? My monitors are fine with all of that, which suggests the x1900 is fine with that also. I have made some progress in that I have outputs 1+3 on the TH2G working (with or without 1920x1080 on the other DVI out), but nothing apart from the local monitor wants to display the centre 1280x1024.