This may not be your problem but I have an A4000 desktop with a socketed Superbuster. Anyway, the plastic plcc chip holder has warped over the years so that it doesn't make good contact with the middle pins of the chip (i.e. those furthest from the corners) and presto, no expansion boards show up. If I take the chip out and reseat it it works for a while but then eventually all the expansion boards disappear again, except the accelerator of course. My solution is to replace the Buster socket at some point. So you might try there.
