A1200 keeps rebooting

[Vulture]

Consider the possibility that either the data or the power cable of your hd are no longer in pristine condition and they may fail after a while for various reasons. It's the cheapest thing to replace before researching other options.

[Vulture]

There's a light possibility the accelerator itself is beginning to fail. I had that problem once and I had those dreaded red screens. I hope that's not your problem as those red screens can appear for a variety of reasons.