02 Ford Focus: Crashing speedometer and other oddities

I have an 02 Ford Focus, a bare bones LX with a stick and power nothing with just a bit under 80,000 miles.

I had a little trouble with misfires and electrical oddities but I thought that was behind me with a new battery (60,000 miles) and a rebuilt alternator (70,000 miles). However, periodically my speedometer goes to zero regardless of speeed… it sometimes sticks there for a while, sometimes comes back up quickly… sometimes there’s an obvious event that triggers it (bump, rev in the engine), sometimes not. Seems to happen more when I’m using lights and the temperature is colder and more wet, but it’s not a really clear pattern.

I’d just ignore it and go on my merry way but two times now at stop lights (in cool wet conditions if that matters), all the lights on my dash have come on, as if the car had stalled. I don’t actually think it does stall by the sound when I turn the key, but I’m not sure anymore. The last time this happened I tested… letting out the clutch and giving it a little gas and nothing… until I did clutch-in and turn the key. What the heck is going on?

As a side note, it was suggested at 70K with the alternator that I replace the serpentine belt, but it was a “good time to do it” suggestion, not that the belt looked bad, so I didn’t… related?

As a second side not, I’ve gotten a check engine for coolant temperature, but I think that’s just because I overfilled the tank… I’m going to eliminate that variable soon, but it’s still technically in there now.

Any ideas/suggestions appreciated.