<Sigh> I will never agree with this method of a "fix" You already have a new power board, this leaves only the connector to the power board, the Z connectors, the connector at J210 of the CPU and the J210 header pins 4 and 5 as potential problems.

Reflow J201 pins 4 and 5 and re-punch the CPU power IDC connectors at all 4 points and you will likely be fine.

WARNING: a rant is coming...
FWIW, my STTNG is running on the original 25 year old PDB, with the original capacitors and all original wiring ... including the Z connectors that a lot of folks bypass. The machine was engineered to run this way, and it's not a design flaw as these machines didn't random reboot when they were new, and there is no reason they should require a hack to work now!

This is usually a ribbon cable issue, I would double check ALL of your ribbon cables.

Not really, you have a quick disconnect that's connected to nothing, there should be no quick disconnects on the ground lines and the picture doesnt show where any of the speaker panel ground connections land.

My machine only has a single ground on the speaker panel that goes to the DMD display.

Your game likely has corroded headers/connectors, possibly from exceeding their life expectancy; every time these connectors are taken off and replaced is two mechanical actions against the life expectancy of the part. Both the connectors and header pins were rated to have a life expectancy of 50 cycles...any more than that and the parts plating is expected to be compromised.

The permanent fix is to replace the parts you are reseating.

This is not a fix, but it should get you to the next spaceport.