Just started my car up after my first engine rebuild.
It starts just fine with the normal AWS rpm's but after a while it drops down to about 1200-1400rpm and starts to fluctuate.
When I listen real good, I hear something that sounds like air leaking from somewhere beneath the UIM, could this be the reason for the idle problem?
Also, my new boostgauge shows vacuum at idle, is it supposed to do that?

 

Do a Search, plenty on this.

 

Yes.
Do a search for fluctuating idle or just idle or even try vacuum leak. You will get plenty of topics to read through that will most certainly help you.

 

of course you vacuum at idle

 

Did you get a new metal LIM gasket? Stock was paper is useless after 60k mi.

UIM gasket is metal but if it's worn, you may get a leak.

Sounds like a vac leak, possibly in the gasket somewhere.