i have a 96 dx hatch that i put a z6 head on i am using the stock ecu the car runs fine untill it warms up (goes into closed loop?) then the idle starts surging and it wont rev past 5000 rpm the plugs look black and there are no codes being thrown any help greatly appreciated

 

we are having an air/fuel occilation after car warms up. no check engine lights or anything just a very rough idle and a lot of spitting and poping when revving up, no load.

just got the car running, so i will update after we put some time into figuring it out.