Idle problem

I put a new engine in my 91 talon with a 16g small turbo and tubular manifold/exhaust system and when i start my car it sits at 2 grand till it warms up, then it'll drop down to 1 grand and the idle will go down to about 500 rpms the back up to 1000 and back in forth back and forth between 1000 and 500, why would it be doing that? and how can i fix it?

there are screw on the top of the tb ( biss crew ) adjust it.