Ok, so yeah, your core speed is 765Mhz, but when not playing a game the core speed is dropping to 400Mhz, which is right, because if you had only one monitor, the core speed would drop even lower to 157Mhz or something like that. You can test this by just disabling the 2nd monitor briefly (Show...
Try making sure that the core/mem speeds is running at 400/1050 for dual monitors. I know that mine idles with one monitor at 157/300, but that's not fast enough for two monitors.