Hello,
Did you carefully checked the pcb, i mean naked without vrm heatsink, to check if there are any broken smd component, especially near the vrm power stage.
Check also if the vrm metal shroud, fan assembly, heatskink are well mounted, avoiding any shorts.
From how you describe the issue it seems that, if the t° are not the issue somewhere, applying a mechanical stress on the card by disassembling it, brought it back alive for a few mins.
It could be there are some small solder crack somewhere under the balls of the die or the ram and/or somewhere else on the pcb. That why disassembling could made it works for some time.
The other explanation could be a power supply issue, but i suppose your replacement gpu is quite small and do not draw the same current as the 390. So you could maybe not notice the issue straight.
Usually if you want bring back dead gpu, when there is nothing else to do, you bake it in a oven at a precise t° or use a heat gun to heat the pcb.
The idea is to melt the solder to repair the small cracks, that break electrical connections. This could work in some cases where the gpu have no physical or component damage.
Other than that, it could simply be that the die/ram itself get some high voltage and just died, but pcb is intact.
