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The second repair was an eMac, a G4 mac in the old iMac style. The kept freezing, not the usual fault screen, it would just stop. Software problem perhaps? I tried my usual trick of booting up to Ubuntu to bypass any install OS (and related problems) and this seemed to run fine for a few hours. This machine was donated as a faulty unit, so the data on it was to be wiped, and we didn't have the admin password, so it seemed a good opportunity to reinstall.
It would have been supplied with a CDRW / DVD combo I believe, and Ubuntu picked it up as that, but it didn't like any of the DVD's I tried, and would spin them around for a bit then eject them, so I installed Tiger from an external DVD. This was looking good and ran for a few hours (after doing what seemed like and endless series of Java updates), but then froze again. I did think it might be overheating, but it froze again shortly after a cold boot the following morning.
I knew I had to replace the optical drive, so I dismantled the unit, and gave it a good clean. I took the opportunity to test the memory in another pc - that was fine, so that wasn't the problem, and I removed and DBANed the hard drive to clear the data.
Whilst it was apart, I noticed some of the capacitors had bulged, and when I looked further, five of the nine 1800uF 6.3v capacitors were either leaking or bulging. So, to save possible future problems, I replaced all nine. The other caps on the board with from a different manufacturer, and seemed intact, so I left them alone.
Once reassembled, it booted up fine (I'd swear it was also faster) and has been solid so far. I've even upgraded it to 1GB and installed Leopard.