Idleing problem

I recently droped a 96 ls motor in my 1991 integra when I purchased the motor the wireing harnes was cliped so I had to switch my intake manifold and throtle body so I could use my wireing harness. anyways the motor starts up fine and sounds great and real peppy but the problem is it jumps up to 3000 rpms and sits for a second and then is drops down to like 700 hundred and tries to idle and then it dies and I can't figure it out if anyone has a sugestion or a solution write back Jsidmore@msn.com Thanks Speed'nDA