99 S-10 blazer, service light flashes at 30mph
 

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?