RAM issue in iMac (beeping at startup)

When I was troubleshooting my iMac back in April, I noticed that one time, the computer wouldn't boot and would beep once every 5 seconds. I looked it up on Apple's website, and then just removed the RAM and reseated it. No issues after that.

But then, recently, I turned on my computer from it's turned off state for about a few months. And it beeps again. Obviously, I repeat the same procedure, except that this time, I need to not only do it twice, I needed to carefully clean each pin inside my computer AND on each of my mem stick. Except that after I did that, if I moved or plugged in something inside my computer, it would randomly re-create the same error. If it isn't acting like a wounded animal, I don't know what is. Not only do I not have no patience of reseating the memory sticks every five seconds and on top of that, I'm a generally paranoid person and hearing beeping like that scares me to death.

Any tips? Also, is it normal that it gives me the error considering I have FOUR memory sticks installed? Would it naturally consider that there isn't any RAM if one of them is just slightly off? Thanks in advance for all tips. PS: Late 2009 iMac 21.5", 12GB memory, memory slots from top-left to bottom-right: 4GB, 4GB, 2GB, 2GB.

 

I have no idea if the RAM sticks are original or not, I bought this computer used in 2013 and haven't upgraded the memory since. The only thing I did upgrade is the hard drive.

Okay, I'll try. Any preferable way to clean the slots/sticks?