Ah ues, the treaded toshiba. That make also brought me to here many years ago. I also bought a new mobo, and rebuilt with same non working result. I was eventually explained that many of a toshiba have been brought here, to not ever be fixed. I was eventually convinced to toss it out and get another brand of pc.
I understand that this is not good technical advice, but I have also experienced a bad toshiba and it was never fixed. Without a circuit tester, and a schematic, I do not think you would be able to determine the cause.
I tend to think it is a grounding issue with the case. The case is the only thing in common. If you are brave, attempt to build it OUT of any case (I have used legos to isolate the circuitry from the table), and see if it runs out of the case.
Or, keep the harddrive and have any files that may exisits transferred to a new device is my best advice.