Does not idle down?

I had a question. On my 72 240z, It idles at like 1200 now which i can live with but when you press on the gas it takes like at least a min. to Idle back down. Normaly it stays at around the 3000's. And it gets kinda shaky when you are driving it around and come to a stop real fast, most of the time it ends up dieing. I just don't know why it takes so long to idle down??? HELP!! Could it be a vaccuum leak, possible in the smog pump hoses? because i know they are letting in air, I had a set of plugs made to plug them but haven't gotten home to try them yet. Do you think this is what it is? THanks guys