iMac (mid-2010) from Japan - memory problem

Hello all,

Long-time lurker, first time poster.

I purchased this iMac (new) in Japan a few years ago: 27-inch, Mid 2010, 3.6 GHz Intel Core i5, iMac11,3

All apps and OS are updated to most recent versions. Running no virus software or extra extensions.

It came with 4 (2x2) GB of Samsung RAM: DDR3, 1333 MHz

It's getting on in (computer) years, so I thought I would give it a boost. I gave the dude at the computer store my specs and he gave me 2 4Gig sticks of Corsair memory (labeled Mac Memory). I installed it below my original Samsung and the computer booted fine but applications started to crash after a few hours (something it had pretty much never done before). I ran memtest and that crashed the whole computer. I tried reseating the RAM (same result), using only the Corsair (wouldn't boot at all), and other variations with bad/no results. I went back to the store to return it but, because I had taken off the stickers, they refused to accept it. Okay.

So, I bought RAM (again 2 4Gig sticks) by another maker. I had heard that iMac users had had good results with Mushkin brand, so I got that. Unfortunately, I had exactly the same results as the above. I then tried using both the Corsair and the Mushkin (only got to the grey apple, no spinning startup wheel), only the Mushkin (kernel panic on boot), etc. I wasn't foolish enough to take off the sticker this time, so I guess I can waste another few hours to return it.

As it isn't likely that all four sticks are defective, what do you suppose the problem might be? As I purchased the iMac in Japan, it came with an odd power cable - it doesn't have a grounding plug, rather, it has only two prongs (instead of two and a ground) and a green grounding wire. I have been using it / this iMac since I returned to Canada for almost eight months with no problems, but I wonder if somehow the power cable is affecting the voltage of the RAM? Apologies if this is a bad guess. Should I bother trying to exchange the RAM for another brand or is RAM other than Samsung incompatible for some reason?

Thanks for reading. Any help appreciated!