99 blazer with vortec 4.3 engine. Suddenly chugging and service engine soon light flashes at 30mph and up. Seems to run sorta ok at idle, and will start and go. I have changed the cap, rotar, and plugs. The EGR seems ok. Any thoughts? And please don't say spider assembly

 

A flashing CEL means that a potentially damaging misfire was detected. You likely have a misfire code set as well. P030x - Random/Multiple Cylinder Misfire Detected

 

I took the truck to the auto parts store today and they pulled PO303 code for #3 cylinder misfire and another code for a Evac selenoid malfunction. I bought new wires to try that. We'll see tomorrow after I change them. Not sure about the other problem, what does it do to the truck?