I'd shut down, unplug the computer and remove the CPU heatsink/fan (HSF). Make sure the top of the CPU and the bottom of the HSF are cleaned thoroughly of old thermal paste/thermal pad material, then apply new thermal paste and remount the HSF so that it fits snugly. It sounds like you're most prone to freezing/crashing when you're running the CPU hard, which makes it heat up; and the HSF isn't drawing away all the heat generated.

The other thing is, how big a PSU do you have wattage-wise? Your components use quite a bit of power, and if you only have a 350W PSU it's going to let you down, especially if it's an off-brand. ECS is not a mobo brand that inspires confidence in me, either.