No Bernard it is not the plug - that is on the lead and I have the identical symptoms with two different leads. It might be the socket but I'll be surprised if there are any soldered connections between the pin contacts inside the socket and the other pins that come out through the PCB.

Your idea on the BIOS sounds viable though. I must have a look at that (during a period while the connection is good) - but the snag is that if I choose a different option in the BIOS, then the AGP slot will be disabled, and if the PCI card does not work then there will be no way to see the screen options to setthe bios back to addressing the AGP slot.