My PSU does 1200W what should be more than enough:
6*170W = 1020W + 30W (CPU) = 1050W makes 150W for fans, mainbaord and backup.
Downclocking my GPUs to 500/300 before mining, to reduce power consumtion, doesn't work neither.
Any ideas? Why does the one pcie 1x port work, but not the other one?
Your forgetting that your 1200W PSU if its 80 Plus certified is only good for 80% of 1200 watts. Bronze, silver, and other ratings give a little more. Say you have a gold certified at 86% thats only 1032 Watts
I ran 2 5850's and 1 5870 and had a PSU fail to work at 850 watts, needed 1000 watt PSU. You just dont have enough in your PSU in my opinion.
You got it backwards. 1200W means it can deliver 1200W to your components, and will draw even more from the wall because of inefficiency.
In addition, most branded PSUs will happily deliver 100W-200W more than what they are rated for as well before over current protection kicks in. What you lose is some efficiency (80+ Gold PSUs for example may drop to like 80% efficiency when overloaded).