100 C is way too hot for those cores, you may have damaged them already. Crank the fans to keep temps. not much above 70 degrees C.
I think he was referring to the VRM temps being 100 or below. If his cores were at 100 they'd be throttling themselves if I'm not mistaken.
Umm, did a calcuation and if you have a reference 5970 and reference 6990 (not overclocked) the total power draw for both cards should be 600W. That does not factor in other power usage by the system. So I'd say your power supply should be up to the job as long as it's not faulty in some way. When you used the dual 5970 did you also use the same power supply?
Also keep one or all the monitors on and see if there is a blue screen (drivers crash) before it shuts down. Try also leaving the 6990 in the non overclocked position (it's got a switch on it - or it should).