Jerking

I have yet a nother problem to add. Ive only had the car 4 days here is whats going on. In low rpms the car wants to jerk and studder and wont stop studdering untill 3,000 rpms or so. And when i get into 3rd gear it accelerates hard for a second or so then the car jerks very hard almost like i slammed the breaks on. It has always studdered in the low rpms since the day i got it but i just added the HKS(BOV) today and since then that is what has made the 3rd gear jerk so hard. Any ideas anyone.