Macbook constantly restarts & won't boot from CD Drive

Hello!

I have a previous generation 13" macbook with the aluminum case but no graphics card.

Anyways, it recently started not going past the grey screen and it wont stop thinking. I tried booting from the CD drive, and once selected, it just restarts rather than booting into the CD drive.

I did all of the resetting of the Pram, holding down the power button for 10 seconds, etc...

So, I tried removing the hard drive and putting it into an external enclosure, then with my Macbook Pro, loaded Leopard OSX onto it after erasing the drive.

The method I used for loading the OS was simply putting the Leopard CD into my MBP drive, and then going through the normal install process after restarting but selecting the external drive rather than my MBP to install it on. It appeared that it went well, and even restarted, booting into the external drive. It wanted me to go through the setup process.

However, when I put it back in the Macbook, it now just constantly restarts before you see the apple logo, and refuses to even give me the option of booting from CD.

What do you guys think? Logic board? Motherboard? Or should I try something else?

Thanks,

Andy

Any takers? I tried removing one stick of ram at a time this evening to see if that would do anything. It didn't.