Hi Nick,
I can understand your frustration. The following is my list of recommendations, I understand a lot has already been done already but I feel it may be worth going back over some stuff again.
Thermal paste - insuring that this is spread out evenly and properly is important
Fans directions- check to insure the fans are set up correctly, insuring that the flow of air is correct (sometimes fans may blow air in, may not be blowing air out of the system).
Fan speed - check to insure that settings are not set to minimum, if fan speed is low you can almost expect slow fans to fail at bringing in fresh air or removing heated air
Some questions on my mind:
When exactly is your PC shutting down, you say it "suddenly auto shuts down" but when exactly is it doing this, on BIOS? after a few minutes? from the word suddenly one can only assume that no changes has been made to the system and this auto shut down has resulted
You said you have replaced a lot of equipment, what equipment has not been replaced at this point, what about graphics card, is there a gcard plugged in and have you double checked that the fans + heatsink is correct on this too? or what about fans, you talked about the h60 and h110i, but what about the case fans? are the air intakes clear (dust free) and all fans working correctly?
please be aware, that while we do our best to help, we need to work with what you answers you provide.
the PC auto-shutting down is an expected system response due to over heating. So my mind is on the following: fans not working, fans functioning below normal, fans not circulating air correctly due to misconfig, heatsinks not seated correctly and unable to absorb heat, heatsinks becoming dislocated (this has happened to me on some graphics cards where the heatsink loses contact with the surface of the chipset), inadequate ventilation (inlets/outlets blocked outside or inside the case)