I got a 3COM 3c509b NIC that I got running in a 486 VLB motherboard (MS-4132).
I got it in a pretty rough physical condition, but after cleaning it and removing some rust with vinegar I got it working fine.
I used it for a couple of days to transfer files via ftp, and even got a ROM chip with XTIDE working to detect my SD card, but somewhere along the way the card stopped working.
The indicator light turns on for a split second on power on, but that's it. The light stays off and the card is not detected by the config software and does not boot the XTIDE ROM.
I tried cleaning it again with soap and vinegar, and baked it in a low temp oven (>50c) for few minutes to dry it, and it came back to life! but after few hours of tinkering with the computer it died again, seemingly between power cycles of the computer, and baking it again did not help.

I suspect a faulty solder joint somewhere, but after inspecting the board I could not find any.
The main chip has some bent pins, but after thoroughly inspecting it with a multi-meter I concluded that there are no shorts there.
The L1 inductor(?) felt a bit loose, but there is continuity on the pads, so I guess it is ok?
I also tried to reflow the solder on the voltage regulator (V41) and the MOSFET (Q41) to no avail...

I have a multi-meter and an oscilloscope, what can I do to further diagnose the card?

Thanks 😀