hey thank you for your detailed answer...

hardware part:
- i have 3 * hd 7990 ...
- i looked it up and the system uses only 1,7 gb ram so there is enough left
- i have 3 powered risers and managed to put 50% of the whole system on each PSU with my cable configuration
- nothing gets too hot on my mainboard or on the risers.. i checked... everything feels cool except the gpus of course
- each graphics card has 2 pcie power-cables and 1 molex at the powered riser cable ...
- i have two 1000W bronze PSUs... and each has to do 50% of the work... but i think they are not so good ... maybe thats really my problem ... i already ordered new ones.. my friends rig is working pretty fine with 2 gold1000W PSUs... but i think you are right... it is really tight dimensioned ... 1200 W PSUs are way more expensive ... lets hope the 1000W gold ones do the job..

software part:
- i tried the 3 different settings for all my cards at once...sorry for the bad explanation but of course i have the same setting for every card... but i found the best and most stable thing is this
--scrypt -I 13 -w 256 -g 2 --gpu-fan 100 --thread-concurrency 8192  thats really the best setting for this card (dont knwo why -g 2 but it is rocking i can tell)--> my friend runs the same rig really stable with this config and 3 * radeon 7990 at 4190KHs .. i think thats really nice... and with undervolting to 1055 nothing gets to hot (nothing is over 79°C and some gpus are at 62°C)!
- i think it is no driver problem because i reinstalled and tried several versions of the catalyst and appsdk drivers... and i did it carefully and correctly


thank you very much much for getting turned on by my problem
i report back when the new PSUs are here... you confirmed my assumption with your really nice list... thank you again