I can run 5 GPUs all day long, but if I try mining on 6 GPUs, the system turns off in half an hour. The power LED blinks and case fans keep spinning, and I need to flip the power switch off and back on to get it to boot. According to multiple places AX1200 and 890FXA-GD70 can handle 3x6990. The GPU temperatures are always very good (under 85C). I have underclocked video RAM to 350MHz. The machine has a Sempron 145 CPU and one IDE hard drive. USB instead of IDE doesn't help. Do I need a PCI-E extender or is there an easier solution?
Based on available data, a single 6990 draw close to 400W, mainly on the +12V at full load. With 3 cards, you're looking at 1200W which is the rated limit for the AX-1200. Add in the power required by the motherboard, CPU and such, not much compared to the GPU but as you discovered, just enough to tip the over-current protection. Downclocking the ram chips don't really help much as they generally consume about 1~2 watts per chip, halving that makes very little difference.
It's usually not a good idea to push a PSU at close to 100% for extended period of time, dies faster and therefore reduce your real income from mining. Getting a "cheap" auxilliary 450W PSU just to connect a couple of those +12V connectors may be a better long term solution.