Isuzu Rodeo Sport 2002 check engine light problem

My Isuzu Rodeo Sport 2002 4 cylinder 2.2 Liter engine with 40000 miles has the check engine light appear when I go above 3500 RPM. After restarting the Rodeo the next morning the check engine light will go off. I have had the car in for service before and they opened the valve cover gasket and cleaned out the fuel injector and exhaust assembly. They also replacd the EGR valve that they reported was sticking. This was nine months ago and the check engine is reappearing once and a while. The car runs fine, no problems, no loss of power but the check engine light is a pain. When I have taked it in for LOF or other minor service they clear the check engine light and tell me the error code is due to emissions. Can you help me.