Do those board ground mods. Sounds like you did your due diligence!

What's the exact voltage at the npu cap? SoundS like a connection issue honestly. Unplug the sound card, driver board & see if the npu and display boot

Can you bump the voltage up a bit so you have 5.5vdc at the caps? My monte Carlo doesn't boot well unless I'm reading 5.25 or so VDC.

It could be bad roms as well.

You don't need the reset board attached. Once the piggy deux is installed and the rom install on top of it, you don't need to remove the piggy deux ever. It should be a main staple because it bypasses the fucking socket from hell.

Also when you say piggy deux you mean this one from pinitech right?

https://www.pinitech.com/products/gottlieb_piggydeux.php

https://pinside.com/pinball/forum/topic/swemmer-gottlieb-system-80b-mpu

You could buy this and new roms and see if that changes anything. I would buy your old mpu from you. I have the test rom and rig.

Any chance you could post some photos of the display and error garbage? It could be helpful for future tech.

Also can you post photos of your mpu? Front and back.

Check list: ground mods, install piggyback deux, remove backbox light, proper power at PS, proper power at mpu, rebuild interconnect, new roms, check for mpu defects near riot chips, check/clean connectors.