Aveo 2006 Misfire

[rem300wm]

Hello to all
Well the timing belt went out and we put a used engine as a replacement,here is the fun part,after the swap the engine light started to flash,code PO 300 ,engine misfire cylinders 2 & 3, well after replacing the plugs,wires and the coil,the light still flashes but it does it after the car runs for a while and gets to about 60 miles x hour.
The weird part is that after turning the car off,the light does not come on until it goes again over 60 miles.
And the car runs smooth,no shaking or nothing, just butter.
Any ideas to try?
Thank you and best regards,
Marco.

[Eg.h2o]

What year motor was put in?

[rem300wm]

Hello to all
Eg,
engine was put in a week ago,and according to the yard that sold the engine to me it was pull from a 2006 Aveo LS ,just like mine.