The problem with bad solder joints and cracked traces is that they are often
difficult-to-impossible to see.

We recently had a similar problem that was traced (joke intended) to a
cracked trace under an IC.

I would start systematically unsoldering and resoldering every joint on the
board. It's not enough to melt the solder and push it around. You need to
suck or wick it off, then apply fresh solder, preferably eutectic (63/37).