Service Engine Soon Light PO300 code

I'm getting a code PO300 code. I just replaced the front & rear valve cover gaskets because Rear gasket was leaking badly. When removing the upper intake I labeled all connectors and hoses plus took photos and everything went back together just fine. I did remove all the spark plugs and cleaned them and checked the gaps for 43 thousands. All the coil packs were dry, no oil on them and the plugs seem to be clean with just a little blackness on them but are a little worn. The code PO300 comes on only after the engine warms up and sits there idling at 750rpm. The engine runs smooth but in cycles, smooth for 2mins then a little rough for 20sec then smooth again and That’s when the code gets triggered. If you hold the throttle at 1000 rpm or more it doesn’t throw a code and runs smooth. Tomorrow I'll check the egr pipe and gasket that hooks up to the rear of the upper intake since I reused that gasket and also install some new NGK plugs. As I said, before I did the valve cover job there was no PO300 code. My question is there anything else I should be looking for? Thanks