I've reflowed every joint on that board (literally, all 2000+ joints), just in case. Still getting a Sad Mac error unfortunately. I'm not having much faith in the future of this logic board.

Click to expand...

That's a lot of work (and probably unnecessary).

If you haven't already read the lone pdf document at http://68kmla.org/files, you might want to. In particular, check out the section on the "whup-whup-whup" sound, which might be the same as your "flup-flup-flup."

Note for future troubleshooting: Take some basic measurements very early on in the process. Soldering 2000 joints should be far, far down on the list of things to try (a bit after "transplant spleen").