running rough

Here is my confusion..... I put a 5.2l in my dodge ram that had a 5.9 in it but now if I have all the vacuum lines hooked up it dies out like the rpm gauge drops and when i disconnect any of the vacuum lines it runs better a little rough but doesn't die out. When I hit the gas it revs up without hesitating at all and drives fine only when I let it get back down to idle in 1st gear or in reverse when stopped it dies. It is getting better since I ran it for a bit and kept hitting the gas to keep it running but is still dieing when you just let it sit can someone explain to me what is going on....