EGR Valve opening 94 Z

My 94 Z starts running rough at 1500 rpm under load no problems at other rpm. Chev dealer said the computer was opening the egr valve 100% because of a lean condition which made the engine run rough, they cleaned the injectors, replaced the fuel pump and regualor. Picked it up and it still had they same problem. Dealership said drive it awhile and bring it back. Anyone have any ideas what my problem is. The computer is not setting any codes. Egr valve and sensor are working Ok according to the dealer. Help.

Thanks
Mark