Help starting hardware test on 2006 intel iMac

My 1st generation Intel iMac has been giving me trouble lately. It has been freezing, going black, or shutting down frequently. This computer had been trouble free for the first 5 years of use, then the hard drive stopped working, then started working again when I decided to give it a go at replacing it. I ultimately did replace it myself with a solid state drive and have zero regrets doing so. It booted up first time and hasn't given me any trouble at all. The hardest part about replacing it was getting the case open. I've run the disk utility against it and it checks out OK.

I have also put a 2gb memory stick into it, as one slot never worked. The spec says this shouldn't work, but it has worked fine for a few years now.

I'm running Snow Leopard 10.6.8 as that's the highest this computer can take.

I'm trying to start the hardware test, which I have on the original install disks, by holding the D key down during a restart per instructions but it isn't working. What else should I be doing? Is there another way to run this test from a download?

Pop in the AHT disc, usually disc 2, reboot and hold down 'c' immediately after the chime and keep held down until the gear wheel starts. Holding down 'D'applied to later models released with Lion and/or Mountain LionTry taking out that memory stick that was not compatible and see if there is any difference.
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Using Apple Hardware Test

I'll try C but even the original booklet instructions say to use D. As for the memory, I'll give it a try as well. Maybe just resetting it in the slot will work.

Given that I disassembled the whole thing to change the internal drive, and I used a solid state drive at that, I've always been surprised that it worked at all! But it has for a number of years, so why should I be having problems now? I replaced the only moving part, (I guess the fan moves too, but I can hear that working) but I guess things don't last forever.

And here's another observation, this started happening after the Flickr upgrade. The new Flickr is a total resource hog, with infinite scrolling of thousands of pics. I wonder if I hit a high memory address that I'd never otherwise use on this lightly used computer? It's a remote possibility. That's why I want to do the hardware test, to see if I have a bad memory address.

One item to consider.

Just had the same issue on my wife's '07 MB. Shutting down, randomly going to black screen... and no kernel panics nor significant crash logs.

Her issue was the fan. Have you noticed the fan spinning at high speeds for no apparent reason or not noticed it spinning at all? I tried first going in and just taking a can of air and spraying out the collected dust - that worked for about a week and then I noticed the loud fan noise on initial startup and even though it wasn't freezing/shutting itself down any longer and replaced it. Been over a month now and problem has not returned.

Still, wouldn't hurt to test the memory.

I did notice that the fan was running. When I put in the SSD, I noticed that the machine had become pretty quiet, but recently the fan had been running. I can check that out, but it requires opening this beast again.

Yeah, and why I would run the hardware test and test the memory first.

The combination of a 6-7 yr old machine and being that it is an iMac, I wouldn't even open it up without a new fan in hand. I found one on Amazon for the MB - looked to be a new stock fan <$20.

Just the fact of it shutting down/going black and no kernel panic... could very well be the cooling.
Could be some other more expensive item also, but fan is where I'd start.