Overheating might be your issue. Try opening the PC's case and play like you do normally. When the slowing down begins try and experimentally touch the metallic part of the CPU heatsink, then try touching the graphic card's, and then the northbridge's.
If one of them is really hot, I mean if you barely can keep touching the metal, you might need a better heatsink there.
You can, and should, substitute your senses for gauging the temperatures with the probe of a small digital thermometer, for more accurate results. CPU's should never get over 50° C, same for the GPU's of the graphic cards, and unless there's little to no air ricircle or you're attempting overclock, it's very difficult that the northbridge should get even near 40° C.
Another problem might be caused by the inaccurate use of the thermo-conductive paste between CPU and heatsink in assembly. If when the heatsink was put on the CPU said thermo-conductive paste was placed badly, it won't allow for a good heat conductivity between CPU and heatsink (this is to be tested by someone who knows how to dismantle and remount the heatsink on the CPU only), the heatsink may seem not so warm to the touch, but it's because it doesn't receive enough heat from the CPU.
Try installing a softare that reads the temperatures from the integrated heat probes on the motherboard, like Motherboard Monitor or something similar, for a more precise temperatures reading.
For last, try to gauge the heat of the air that is moved through the power supply by its fan. If it's very hot, you may need a more powerful unit.