That is interesting. League of legends doesn't require a whole lot so you should be able to easily max out the settings and get 60+fps. Either way, without a doubt, you should use the PCIe power connecters. I think the 770 may have two power connectors. You'll need to plug in both of them, I'm thinking its an 8 pin and a 6 pin, can't remember for sure, but the power supply will have them labeled. I'm POSITIVE that's what you should use. The issue lies elsewhere....could possibly be a bad PSU, but nonetheless to answer your question--use the PCIe power connectors.