maxdan

Hey guys so I have an S14 Zenki with the SR20DET Black top Engine, I just changed my differential with a R200V Diff but used my old Welded Flanges on it because I had no other ones. So I was driving out of a Safeway parking lot and while in gear like 1st when I started releasing gas so it was rolling in 1st gear it started jerking back and forth, so I put it quickly into neutral nothing happened. Then driving out I put into 1st accelerate then 2nd gear, and it did that once again jerking the car back and forth while Black smoke was coming out of the back. So I parked my car , went to a friends house came back , started my car. At that point it seemed to run fine just standing there, so I decided to try and drive it home, because I don't wanna leave it in a abandoned parking lot overnight, and surprisingly I drove home totally fine, my car wasn't jerking or spitting out black smoke. Can anyone help me out with what this problem was. I havent drove my car ever since , like 2 weeks. I thought it was maybe the new diff but I dont know for sure, can someone give me some advice on what this might be thanks.