My guess is the fan,
witch is a part of a safety circuit so the cube wont overheat.
 I think this circuit measure the inner resistanse of the fan comparing it to the 12v rail. when its a mismatch the safety circuit cut the power to the relay and its shut itself off.
When you hit the overclock, the 12 v voltage is dropping (maybe some mV), the circuit is measuring the 12v regarding to the fan current (which is the voltage indirectly and therefor the innerresistance, conf ohm's law, U=R*I). If its a mismatch it turn itself off.
it seems to me that the electrical tolerence of the fan and the safety circuit is very sensitive. Sometimes when i touch the cube i can hear the relay is clicking.
You can test this just to disconnect the fan when the cube is running or partly block the inlet/outlet of the fan so it changes its rpm, and the circuit will switch the cube off. Of you can try with another fan with the same electrical caracteristic. that will maybe solve the problem.
i believe the tolerance for the "fan-part" of this circuit is very small, maybe within a few mA.
A really dirty solution is to bridge the contact point on the relay. But you will then not have a safety circuit in case of overheating!!!

thank  you guys for good tip.
I will try this to see what i will choose

Thank you 

i believe the problem is the failsafe unit in the cube. The clicking noise you hear is from the relay witch give power to the blades. As a part of a safety mechanism this failsafe "unit" is sensing the current to the fan, so if the fan stop the current will change and the relay click off due to not overheating the blades. You can try this to either change fan or simply to block a part of the fanoutlet with a piece of cardboard so you hear th fan is changing speed and sound. i think you then will get a different , longer, shorter "uptime" from your cube. A bad way to solve the problem is to bypass the failsafe circuit to bridge  the two terminals of the relay. This is the worst way to solve the problem and  beware of overheating.!!

A total newbie, with a empty wallet who would like(need)   to enter the lotto.

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Thank you

Yeah, i have but its to much info. a typical scenario is problem due to drivers for dif miners, ie antminer due to miner programs and so on...

Does any know whats is the best linux distro for mining bitcoin,litecoin?

This is nice. 

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Thank you.

What i did mean   with a "smaller version" was regarding to the speed, 100Gh/s, 256 Gh/s

Lookin good.
Do you plan to produce any smaller version in the nearest future?