System booted after moving RAM slots

[heath1]

A few days ago, I turned on my computer and it hung on the ASRock BIOS splash screen. I waited for about 10 minutes for any activity and then attempted to go into the boot menus, the system was frozen. I force turned off my PC and tried again 2 more times, same thing. I turned it off again and removed one RAM module and it booted (I had 2 modules in the 1st and 3rd slots out of 4 for dual-channel). I then replaced that module with the other and it didn't boot, so I thought that the second module was bad, because it did not boot the system when it was by itself, while the other did. Then I decided to place both in slots 2/4 instead of the original 1/3 and it's been booting fine ever since. I've had my PC about 3 years. Any take on why this happened? Thanks!
Edit: The system acknowledges 8 gigs of RAM

FX 6300
R9 270X 11217-01-20G
CX600M
Corsair 230T
Crucial Ballistix 8 GB 1600MHz
Asrock 970 Extreme3 R2.0

 

[R_1]

solder fatigue, corroded contact, scratched trace on motherboard, there could be thousands of reason why a slot could go bad.
I would test and bench everything else. what damaged the slot could have damaged something not so easily seen.
the green labeled cx series are suspect, check your voltages, each rail should be within a 5% range +/- if the 12V reading 11.5 it is within specs and fine, outside of 5% either way and you may have an issue, if they are off by more than 5% I would RMA that unit it carries a 5 yr warranty.

by suspect I mean they are of questionable quality.