Thought that I would resurrect this thread to say that I opted to install a new motherboard & lo & behold, the problem is repaired. Since I've swapped an old foxconn for a new Asus p5kpl-am & added more ram, I've now got a new speedy box into the bargain.
The only downside is £20 quid on a new graphics card when that seemed to be the trouble. Still, all's well that ends well.