Stumbling Idle... Searched and have tried everything else...

[Thieu]

Okay, I have a 90 KA24E which runs great, but not at idle.
It have the AIV removed and all hoses plugged up. I just replaced the AAC (behind intake manifold) with one from the junkyard. At idle the car will sound crappy and stumble. It's not horrible but definitely annoying and it's obvious something is wrong. As it idles it will sporatically fix itself momentarily and idle nicely; the idle rises to 1000 as opposed to 700-800 when it's idling badly, and the exhaust stops sounding like its burping.

I have a new 300ZX fuel filter, cap, rotor, and a K&N intake. I just cleaned the MAF and my injectors are very recent.

There's a clear hissing coming from the intake at idle. Is it supposed to be doing that? When the problem fixes itself momentarily the hissing gets louder for a second. I've looked for vaccum leaks and found nothing; I even sprayed carb cleaner on them and nothing.

The only other thing I can think of is a leak in the intake manifold gasket or a dirty electrical connection.

This is driving me nuts... please help me...