Could be a poor connection where the PSU
plugs into the back of the machine...
I know it sounds dumb, but I had a
similar problem on my old 1200 and it turned
out I didn't have the connector pushed in
all the way.
Other causes could be a faulty
PSU, perhaps a "cold - solder" or loose solder
point on the motherboard somewhere...I'm sure
there are more. You could try unplugging your
hard disk and see if it still does it etc.
