It really sounds like you may have already answered your own question. I was a bench repair tech in a past life. I lost count of the number of head-scratching bad behaviors I repaired by carefully resoldering perfectly fine-looking circuit boards. Wave soldering temps are always a compromise between too cold to flow properly and hot enough to damage the components. So, different-sized joints get soldered with varying degrees of effectiveness, hopefully all within an acceptable range. On connectors especially, where the connector pin is heavier than a typical component lead, that mass acts as a heat sink, preventing the solder from attaching perfectly. Sometimes you can see a microscopic crack with bright lighting and serious magnification, wiggling the pins one-by-one.
Check those connections and also look hard at any heat-generating component's connections. It's a repeated expansion and contraction of the solder joint causing a fine crack that you're looking for. The repeated warming / cooling from normal usage, gives you a joint that still looks perfect, but isn't. Instead, it now works as long as the two sides are touching "enough". Then, a little heat or vibration, and poof…
