Idle problems

I got a 94 Civic DX, I just recently dropped in a 94 Civic EX engine. The only problem I'm having is with the Idle. When you first turn it on it idles around 3 grand, if you let it sit for a while, or give it a little gas the idle speed will then drop down and fluxuate between 1 and 1 and a half grand. Me and my dad tried re-using all sensors and everything from the old engine and wiring harness and still no luck...so if anybody has any suggestions i'm all ears. Thanks!!