After essentially hitting the wall and running out of ideas yesterday, my good friend Nicklas pointed out that I had a suspicious solder joint in the lower left side of this photo:
And of course he was right. If ever there was a cold joint :). A quick resolder took care of that, now the signal is at least stable.
Lesson learned: Always check, check and check your solder joints again. A bad solder won’t always come loose right away, but can let go while you are fiddling around with your contraption.
Rechecking all the other joints revealed nothing out of the ordinary…