I have this same problem in my manual 89Z.
It didn't used to be as bad, but the last few days I can barely clutch long enough to downshift before the RPMs drop to below 500 & then die. Even when traveling 75mph on the interstate and clutching around turns, it will go from around 2000-2500 to 500 in a second and within one more second the battery will die.
No idea how old any of the parts are, bought it a couple months ago from a guy in the town over, my first car. He seemed pretty trustworthy though. I drive 18 miles to & from work every day in it, and not being able to shift or brake or idle for longer than a second or two is a pain, to say the least. I was about to go to the salvage yard in a day or two & get a new alternator, everyone I've explained the problem to has said that's what it is, but thought I'd ask on here too.
Thing is it doesn't happen 100% of the time. Usually it's fine for the first 5 minutes of driving, but after that it drops RPMs faster and faster each time I press the clutch until it takes a fraction of a second to hit 500.
Sometimes when starting the RPMs rev a bit like usual, then drop to 1000, then will rev up to almost 3000 and hold for a second before dropping back to idle at 1000 before I put it in gear and pull out.
Also, sometimes my dome light doesn't come on when the driver's door is opened, but sometimes it does. Always works with the passenger door. And once in a while when I crank the ignition absolutely nothing happens. No noise, movement, nothing. I wait a little then try again & eventually it works, but all of the EQ settings and the clock on my head unit are reset every time. I figured this was the starter.. s'what some people have told me too.
Any ideas?