I had an alternator tested at autozone and it checked out ok on their machine. But when it is in my car, all lights flicker/speedo bounces until after 5-10 mins of driving after car warms up. Then if I use the headlights it will sometimes come back. Testing with a multimeter the volts are in the 11 range when everything is going crazy, like the alternator is not switching to high output mode when it needs to, but 13-14 volts when acting normal (warm temp, lights off).

If I unplug the 4 wire plug from the alternator, everything is fine even with every electrical accessory on at once! (but I don't think the alternator will charge the battery without that plugged in)

Also The problem doesn't really happen when it is warm out except for the first 10 seconds after the car is started. In the cold weather we have now, it can take 20-30 minutes until the electrical system stops going crazy, and even then if I turn on my headlights, there's always the chance it will start all over again.

Before I buy a brand new alternator, does it sound like the alternator is definitely the problem since the electrical system is good with the alt unplugged? Or does it sound like I could have a short someplace? Car is a 96 civic hx

(I tried 2 used alternators and it happened with both - bad luck? Also replaced battery & ELD)