Since your computer crashes even when you do a memtest86, and memtest does not record any memory errors, it does not sound like a memory error. When you say "Crash" does it freeze or does it restart or does it power off?
If the motherboard is bad, it will usually not boot at all unless it is a heat problem. Some motherboards have a heatsinc and fan on the front side bus. Does yours? If it does, is the fan still good?
It could be the power supply. If you recently installed new hardware, it could be just over the wattage that the power supply can handle. Try unplugging everything but the essentials to see if the crashing stops... if it does, either one of your hardware components are causing the crashes or your power supply cant supply enough power.
Have you been over-clocking your system? If you have, then there is a decent chance that your CPU is bad. I have personally seen this exact thing happen with a bad CPU. It is difficult to narrow down though. I happened to have a spare CPU at the time. A bad cpu does not have to overheat in order to cause problems.
Just in case, check the temperature readings in the BIOS and your BIOS might even have error logs that you can check.