Tried to edit, but timed out - actually connecting the red cable just makes the mac die instantly with no particular sign. The sad mac is happening more in a casual way - most of the times the yellow cable brings to a nice and reassuring chime, but sometimes (just sometimes, there doesn't seem to be any correlation) it goes to sad mac chime in the process. 
Any thoughts? Could it be a problem with current or voltage regulation? Something drawing too much power? Something else?