You haven't listed any info on your system. Are you using TH2go Digital edition? Did you run the compatability wizard on the Matrox website?
I don't see any 5800 series listed in the compatability list there.
what is your OS?
As a side note, have you not tried 3 screens using the built in Eyefinity? People are waiting to get some first hand reports on that.
I'm running a C2D E8400, 8GB RAM, Vista64.
I don't think the 5800 appears on the compatability list because it was only released on Wednesday this week.
You can't use Eyefinity unless you have either one monitor with a displayport, or an active DP->DVI convertor. I have neither.
What I do have, however, is a TH2GO Digital Edition which was working just dandy with a 4850 previously, but now cannot.
Also, I'm using 3 x LG L227WT monitors, they used to sync at 59Hz with the 4850.
Getting worried just a bit.