Startup Failure

I have posted this question at the MacFixIt bulletin board (http://www.macfixit.com/ultimate/Forum8/HTML/000474.html) but would like to have more suggestions.

In a nutshell, my 5400/180 (upgraded to 6.4Gb HD and 88Mb of RAM) was running happily for months on OS 8.6, but suddenly it just refused to start up, even from the OS 8.5 CD. After many days of repairs, reconfigurations and tests, the problem still persists. Since the last post at MacFixIt I tried to put back the old 2.4 Gb HD with OS 8.5 on it, but it still refuses to start up from the 8.5 CD, and then just refuses to start up from HD. This eliminates the likelihood of software and HD as culprits, but a few technicians suspect the RAM while I suspect the logic board. The latter already had a problem when I acquired the computer which disables the screen brightness setting but never caused any problem, but the RAM causing problems?

I am at my wit's end, so any suggestion would be very welcomed.
SeeleAsked:
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SeeleAuthor Commented:
Edited text of question.
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NFUNKCommented:
With A 5400 you should check the voltage coming out of your power supply to the drive connection make sure that you are getting the full 5 vDC for the electronics as well as the 12 vDC for the drive motor. with the all in one form factor there were several compromises made in the reliability of the power supplies. Also remember to reset the motherboard CUDA chip to try to get a boot.
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SeeleAuthor Commented:
Thank you for the interesting insight; I shall be looking into it. Yesterday just for the sake of it I gave it another try and it restarted from the 8.5 CD clean as a whistle, so I took the opportunity to investigate further: made five partitions, two smaller ones for systems: 8.5.1 (the latest which can start the computer) and 8.6 (I still haven't admitted defeat yet); two larger ones for apps & utilities and documents, and a small one as drop-zone for CR-R burning. This morning I pulled out the original bank of Apple RAM and it seemed to improve by quite a lot, perhaps the two banks of RAM have different voltage and ns ratings and might have contributed to the problem. The 8.5.1 boot partition never fails to boot up but the 8.6 partition can only do it once every twenty times or so; Norton and TechTool Pro2 checked them out and repaired all the usual minor faults, but TechTool 1.1.9 reckoned the 8.6 boot partition as perfect, but gave me a very ugly jolly roger for the 8.5.1 partition! I hope the suggestion re voltage can go some way towards explain the whole 'saga', but in the meantime I have to go through both system folders with a very fine tooth comb. Wish me luck!
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