ZC_powered_4dr.

i have an 89 hatchback with a stock ls motor on 9 pounds with 550cc injectors on hondata s100 my problem is when the car is running between 1500 and 2500 rpm it sounds like its on a studder box, there is a check engine light on and the codes call for iac valve and an o2 i replaced the iac and it did the same thing so i was wondering if yall had any ideas on what could be wrong. thanks