97audiA8Nblk

I just bought this 97 Audi A8 two weeks ago drove fine no issues. One day I got off work and the car started with a all of a sudden misfire and check engine light came on when I get up to like 45 or 50 mph the car has a violent shake as if the thing is going to fall apart. The next following day I took the car to a shop got it checked read I had a misfire on cylinder 4 and 7 got the coils and plugs changed still have this crazy misfire HELLLPPPPPP PLZZ thanks in advance.