Why Underclocking gives better FPS than overclocking a graphics card?
You didnt underclock, you lowered the overclock.
Notice your values have a + in front of them, so they are Base Clock +Overclock. For example, your GPU was 954Mhz base + 424Mhz overclock = 1378Mhz. You now changed it to 954+368 = 1322Mhz. So you lowered your overclock by 56Mhz. Same happened with memory. Your original overclock is possibly to agressive, making the card too hot and throttle, lowering performance. By slightly lowering it, this problem went away.