VY running rough and making noise.

[02vy]

Hey guys i recently got extractors put on the car they were pacemaker 2 1/4 inch it also has a 2 1/2 inch lukey exhaust from the cat back when i picked her up i notice a weird buzzing and knocking noise so they lifted it back up and showed me where it was comming from it looked like the starter motor so i replaced it and turned it back on it was still making the same knocking noise also when im driving and cruising at 60kms if i let go of the accelerator and let it roll the revs drop under 1000rpm almost turning the car off but then it surges back up and if i put my foot down it feels as if its lacking power but suddenly it will just take off like crazy almost as if its charging up. i have no idea as to what it could be any suggestions would help and be much appreciated thanks guys.

Also its an 02 Vy Acclaim V6 thanks.