Something odd is happening to my iMac (2006 24" model - Core 2 Duo 2, 2GB RAM, 250GB HD and nVidia 7400GS graphics).

The machine has been running slower and slower over the last 6 months, now it has started giving artifacts on the screen (random blocks of colour) and it did a kernel panic about 30 minutes ago, whilst I was editing my holiday pics. It managed to recover the database when it restarted, but a few pictures artifacted when I tried to change the exposure (random mauve blotches all over the image).

Syncing it with my Apple TV synced only about 250 images (from 900), syncing again got an extra 100 or so, syncing again and nothing happens.

Mail just crashed and re-opening just crashes and crashes...

iStat is reporting:

Temps:
HD 50°C
CPU 38°C
Ambient 23°C
GPU 49°C
GPU Diode 58°C
GPU Heatsink 46°C
HD Bay 48°C
Optical drive 39°C

Such artifacts are common with an overheating video card in Windows, but the card doesn't seem to be that hot.

And Apeture doesn't, AFAIK, use the graphics card for effects, so it shouldn't have been hammering the video card.

I'm not sure if this is an OS X Lion or a hardware problem.

Edit: The iMac has been turned off for 2 weeks, as I was on holiday, but that shouldn't make any difference... I'll try reseating the DIMMs shortly.

I've removed and reseated the memory (also swapped slots, DIMM 0 now in slot 1 and vice versa). I also gave the ventilation slots a quick blast with compressed air, no great clouds of dust...

I'll see if that has done anything for the stability.

Edit: Okay, that didnt last long, getting artifacts in Mail now.

Try running a Memtest off a Linux live CD

If that's okay then it's unlikely to be your RAM.

Artefacts typically suggest the GPU is on the way out.

Sadly, I'd agree, which means a logic board replacement as the graphics card is part of that.

Can you try a Safe Boot (with shift key held down) to disable any extra extensions that may have crept in? If you have a spare external USB hard drive or flash stick you could try installing a fresh, original system onto it to test if it does the same when you boot from a virgin system.

There's also Hardware Tests, which are on your original System Install DVD, plus Disk Utility to check the hard drive is healthly. As it's affecting a handful of apps, as well as Kernel Panics, it does sound like hardware rather than software.

I'll try the hardware test CD.

Luckily, the 24" iMac has the graphics card on an expansion card, although I don't know how easy that is to replace / upgrade. That was the big selling point, they used a mini-PCIe bus card, which was supposed to become the new standard, but they only ever used it on the original 24" iMac, AFAIK.

Apart from Apeture and MS Office, I haven't installed anything else on the iMac, since I reinstalled Lion. It was so slow after the installation, I lost patience with it and only use it to sync new photos with my Apple TV, everything else is done on my Windows laptop.

Edit: Given the artefacts, I was also thinking it is probably something to do with the graphics card.

Can't find the original CDs, I have Leopard, SL and Lion DVDs, but not the originals... I think they are still in the box, and I have the feeling that the box was thrown out to save space... That or it is in the loft, I'll have a look later.

Anyway, I installed Ubuntu in the BootCamp partition and everything is running hunky dory, if a little slow - slower than I remember SUSE being, but still a lot faster than OS X has been on this machine since I installed Snow Leopard - SL slowed the machine down a lot and Lion slowed it to a crawl, Ubuntu is certainly a lot faster, but ISTR Windows Vista and SUSE running much more fluidly than Ubuntu, but the last SUSE on the machine was about 18 months ago...

i also let it run Memtest86 and the memory passed with flying colours. That said, I've had 3 machines over the years with duff memory and Memtest86 gave each and every one of them a clean bill of health! I'm not sure what Memtest86 does, but it certainly doesn't detect duff memory, as a rule.

Bugger, looks like I spoke too soon. After 20 minutes or so, the artifacts have started appearing here as well.

Looks like it is down to a dodgy graphics card.

In case you don’t know, there is a ventilation slot between the back of the case and the stand. It’s a round hole, and can get plugged quite badly.

Yeah, I gave that a good blast with the compressed air yesterday. No big dust cloud.