Doppel post
edit:
I'll make this post less redundant - when you say the PSU doesn't stay online for more than 5 mins without crashing, what exactly do you mean? Is the A1200 expanded? Its quite common for A1200 to have power problems after the addition of an accelerator, an additional HD or CD-ROM, etc, because the PSU supplied with it is woefully underpowered. Many people resort to using an old A500 PSU or towering the machine.
Or is it just a case of the PSU being dodgy?