I had the same problem with my 1989 4.9. It would miss under light load. It got so bad it felt like it was surging when going down hill. After doing a lot of things to try to narrow down the problem I just lifted and plugged the vacuum hose going to the EGR Valve. The truck ran beutifully after that. I realy haven't fixed the problem since I think Its the EGR relay thats the culpret. I read 45 ohms across the coil of that relay but the two next to it read 90 ohms. I'm going to eventually replace it but its so very cold hear and the truck runs so great now I'm in no hurry. I understand the EGR sends from 10 to 25 percent exhaust back into the intake to cut down on NOX emitions. Can you imagine it constantly giving you 25 percent exhaust on the intake. You would miss too under those conditions.