Yes, that is possible about the bent pin showing up after a while, remember it's all under very high pressure, sometimes if damaged slightly, or slightly out of place, that pressure might final make it bend out of connection or move slightly more.
I's best to check, and let someone else check too, instead of wandering about issues for so long before later finding out it was that.
Have you already, if not, can you test you memory sticks one by one on another computer, to rule each of those out? Test each one over night with memtest86+
And for now, until you figure out the problem, I would not use the 8GB memory sticks, they are not made for this system thus by default would not have anywhere near the correct defaults for X58 (Mainly at tRFC and tFAW)
There is BIOS FF1 on the site, and I have FF3 if you want to try the very latest BIOS that would have all the most up to date improvements and fixes for issues. I would suggest FF3, with updated CPU microcodes put in at the very minimum.
Don't say "I don't want to use beta" that is only left in that state due to age BIOS development was abandoned and those should be considered final anyway otherwise they would be pulled out of beta state due to bugs reports way back then.
This should especially apply now since it's 10 years later and you want to try and use non x58 memory with the board, and are having issues you can't figure out. Ruling out BIOS related bugs would be first thing to check, after confirming no bent CPU socket pins.
I'm not sure what you're calling DIMM0? DDR3_1 (first white nearest to CPU) should be the first slot to be used, and only that slot if one stick is to be used/tested. Slots 2 and 4 should not be used at all unless DDR3_1 and DDR3_3 are filled and this would only be dual channel.
White slots first, and only, until 3 sticks are used for triple channel, then you may use the blue slots, otherwise you can have memory related issues such as you are seeing now, this is by design.