Are the same two GPUs always lazy, or different ones every time?

In 1st case, how do they behave if you unplug the rest of GPUs and only leave these 2 to work? If they work like they should, your PSU is too weak. If they don't, plug them in somebody else's farm. Still no luck? It's a GPU issue, sell them to gamers and indicate the reason of sale to be their poor mining performance — that way you can be sure you're not selling to another miner. Gamers won't notice a thing unless they try to mine themselves.

In the 2nd case, your PSU is almost 100% sure the reason. Mobo and CPU drain about 150W, you only got 580W for 8 GPUs, that's just 72.5W per each, and they normally consume 80-90W each. Get a 2nd PSU, even a weak one, and use an Add2PSU circuit board to hook them up. Power 2 or 3 GPU with it (experiment here) and you should be good.