Can I suggest that you check that everything that is socketed or in slots is propererly seated and isn't loose. Small movements might be making a connection make or break.
In particular check the simms and the processor card, the processor card is on the right hand side of the board, at the front (viewing from the front of the machine). Don't forget to earth yourself before fiddling!
Sorry if this is obvious but loose connections can easily be overlooked.
