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Mum's Mid 2007 iMac suddenly became extremely slow a few days ago. It takes over an hour to boot, and once booted it beachballs almost constantly in every app. Clicking an icon on the desktop takes a minute or so to respond.

It didn't gradually get slow, she must turned it on one day and it was like that.

I have replaced the hard drive with one that has no data yet. I booted off a firewire 800 drive with an OS X installation that boots up perfectly and is fast with another mac, but on this mac it is just as slow as the drive I assumed was failing and removed.

Booting from a mountain lion recovery partition is similarly slow (I never actually waited for it to boot, but it was taking way too long at the grey apple logo when I gave up).

What could cause this?

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Just a supposition : A CPU running on secure mode because it's overheating –  Matthieu Riegler Apr 17 '13 at 12:30
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I agree with Matt. I once took my mini apart and didn't connect the fan properly. The system ran like molasses. OS X will throttle the chip if there is a fault in the fan to avoid overheating. If it can't keep the temps down that way, it'll shutdown. But check towel sure your fans are working or there isn't a fault with your heatsink. –  cksum Apr 17 '13 at 21:21
    
If there was a heat issue, wouldn't the fan be screaming? It's not. How can I check the temperature? Perhaps from single user mode, as that is the only thing tha works reliably. One time it was still booting after 18 hours. –  Abhi Beckert Apr 27 '13 at 20:39
    
Not if the heat issue was due to a dead or obstructed fan. –  JRobert May 24 '13 at 0:58
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1 Answer

Check the RAM.

Ordinarily bad or improperly seated Ram will prevent booting BUT it is worth checking.

However, this sounds like a hardware problem as you have done so much troubleshooting.

My suggestion is to bite the bullet and take the old Mac into a brick and mortar Apple Store.

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I tried reseating the chips. They're a different type to others I have around so I can't replace them. I guess I could try just plugging one in at a time... closest authorised Apple repair ahop has a $150 diagnosis fee :-( –  Abhi Beckert Apr 17 '13 at 18:43
    
If the RAM is faulty or not seated properly, the system won't boot and you'll know because it'll kick off a distinctive set of beeps. –  cksum Apr 17 '13 at 21:17
    
Booting off either of the two RAM chips didn't help. –  Abhi Beckert Apr 27 '13 at 20:32
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