So, I got my mechanic to replace my alternator. When I got it back, every time I started it when wasn’t completely cold, it would crank, fire right up, rev to 1400 rpm, pause for a half second, and then rapidly and abruptly drop to 650 rpm then slowly go back up to 900 rpm. Again this only happens whenever there is already some warmth in the engine. It never did this before the alternator replacement. I honestly should have gone back to my mechanic and said “hey you broke my car, you mind fixing it?” but we literally work the exact same hours, and I have to get a day off work to visit. What should I check?

There is no check engine light, but every 6 months or so it comes on with a p0455 large evaporative emissions leak detected. I just clear it and stays away for a long time. At first I checked the obvious connections, and the duct to the air intake was loose, which I set back in, and it made no difference. I haven’t seen any other loose connections. That doesn’t mean I checked everything but I’ll certainly look more thoroughly this time.