Can't mine with R9 290 + R9 295x2 using 1200W psu

From the specifications of my PSU, it should be enough to mine with both R9 290 (300W) and R9 290x2 (500W):
+12V1 22A
+12V2 22A
+12V3 38A
+12V4 38A

However, when I use Claymore's Dual miner to mine ETH, it restarts every time after show message like that:

Setting DAG epoch #68 for GPU #0
Setting DAG epoch #68 for GPU #1
Setting DAG epoch #68 for GPU #2
Create GPU buffer for GPU #0
Create GPU buffer for GPU #1
Create GPU buffer for GPU #2

I also tried to mine with 1 gpu or 2 gpus. It runs with no problem with any 1 or 2 of them. Can anyone help me think of a reason for this problem? Many thanks.