Loud engine noise and irregular RPMs?

My car will start up with a loud whirring noise that doesn’t go away aside from a few random minutes here and there, driving or parked. It is idling and accelerating irregularly, and the RPMs will sometimes drop in the middle of driving. Say on the highway I'm going 65 MPH steadily; the RPMs will sit at about 2,300 and drop to 1,500. I had similar problems last year at the same time, which got to the point my engine was shutting off on the road. I replaced my starter, so I’m hoping that isn’t the issue.