So finally I managed a nice run, had to raise Vcore 0,25 V for stability to 1,77. Done at 12 x 278 1,5-2-2-5.

GPU was set to 603 / 641 (1282) :

http://service.futuremark.com/compare?2k1=8524023

After some fine tuning I'll go for some higher GPU clocks...........