First let me say, I've assembled about 6 computers over the past 20 years and never had a problem before.
About 5-6 weeks ago I bought a GA-Z77X-D3H MB, an Intel i3 processor, and crucial memory.
I have two sticks of memory and in each of the steps below would try each stick one at a time to be sure the problem wasn't memory. And, since only the things connected were the new parts and the power supply, the only ways I had to tell the board wasn't starting was, the CPU fan didn't start and there was no Post beep.
When I first assembled it (outside the case on a table top) it would not start. Gigabyte tech support said the problem was my 450 power supply was too small, so I bought a Coarair 600 watt PS. No change.
I removed the processor and examined the socket. A pin in the middle of the field was missing. Gigabyte agreed the board was defective so I returned it for an exchange. No change
Gygabyte then pointed at the processor and since every component except the processor had been replaced Intel agreed and I exchanged the processor. No change.
I figured out about here the board has no BEEP speaker (a severe weakness in a case like this).
Since I had seen the fan grunt and ever so briefly try to move I decided to flick the fan. After about 6-7 flicks it started.
Now I have no way to know if it's booting, so I connected a monitor and keyboard. It is booting.
Gigabyte suggested I switch the fan from the CPU fan connector to a different fan connector. Amazing, away she went.
When I added the second stick of memory it went back to the "fan won't start" behavior. I put this second stick in as the only memory and it became intermittent, so Crucial said the second stick was bad. I removed the second stick put the first stick back back in. Again, it began starting reliably.
This was last Friday.
I wanted to be absolutely sure the board was working properly before I put it in my main box, so I probably started it a couple of dozen time over the weekend and ran it for 16 hours constantly yesterday. I turned it off last night at about 1 am. I was about to install it in the case this morning when I decided to add one more start to my confidence and it didn't start. After that it started once and then didn't start the next time. Since then it has started consistently.
To recap, every component in the system has been replaced.
I'm at my wits end. Any suggestions?