1998 mustang v6 3.8l shaking and shut off

my 1998 ford mustang 3.8l v6 starts and all of a sudden the engine shakes very badly and the rpm gauge goes from under 1000 rpms to just a lil bit over again and again but after a few seconds about 05-10 it stops and it feels good again also its been doing it a lot more lately and shutting off but give it a few good revs and its ok and drivable whats wrong with my mustang replies are needed badly and urgently.

that was the first thing i did when it started happening it felt better but then it happened again i dont know whats wrong i adjusted the idle so its at 1 grand it helps with the start up it doesnt shut off ne more but tries to and i forgot to say but it happens alot when the engine is warmed up cold starts are fine but when i go driving then shut it off for like ne where from 5 to 20 mins my baby gets all bitchy n tries to kill her self

well i found out a couple of days ago by my friend who is a master mechanic and he said it probally was the iac so i cleaned it then she ran fine again but i went and replaced it with a new one just for the hell of it