They have 4 connections. Two have 2 x 8 pin, and two have 1 x 8 pin. They power the pci connections. For the risers, they're sata type, powered in pairs.

I think I've managed to solve the problem. I swapped the GPU distribution around to 5+1 (580/560) per PSU. Mining for 8 hours on claymore ethash and no problems so far. 328Mhs with 710W per PSU. Seems about right?