Thanks to yall !!
I decided to give the X750 another chance.
Finally, it worked!!
I really don't know.
Everything - all connection was just like before. I did, however, have temperature problem on my Q6600 after putting my SE610 back in. I decided to reapply the thermal paste to see if I can get the temperature back down to norm. It did. I decided to try X750 again.
This time, no flickering. It did flicker the first time when I turned it on. Then, it just ran fine. On top of that, I can push my Q6600 to 3.6GHz (400x9) with similar temp (low 60s). With SE610, the highest I can go is 3.5G (390x9) or I will get BSOD. Without much effort, X750 ran 3.6G (400x9) - just change the base frequency, and Q6600 obliged. Amazing.
Now the bad news - I placed the SE610 my other PC. Since it's efficiency is higher than regular 80+, I thought I will save some money using it. No matter how I connect it, the PC will not boot up properly. It will boot to the BIOS self diagnosis screen and show the drive list. But that's it. I replace it with Corsair Builder CX430 - no problem there.
It makes me wonder if my SE610 is, in fact, a defective unit. It's just so that it does not exhibit such a really bad issue that causes concerns. But this exercise makes me wonder...
Any thoughts. gents?