2002 Chevrolet Express engine misfire

I replaced engine with a 95000 mile used engine. After replacing engine there is a misfire at mid throttle. Engine runs great at idle and high throttle. I replaced spark plugs and wires, cap and rotor with no change. I have good compression on all cylinders. I tried using the new"used" fuel injectors also called the spider and still no change. I changed the throttle body today because of excessive wear and still no change. Im at a standstill, truck runs good, I dont feel the miss only going on scanner reads and check engine light blinking. Dont know what to try next. Any help would be appreciated!