I have a 2002 Chevrolet Silverado 1500 4.3 liter. I was driving and the CEL came on and started flashing then the truck bogged down and would not go over 25 miles per hour and it galloped. I changed the fuel filter, the spark plugs, the plug wires and I cleaned the EGR valve, checked the pvc valve and the O2 sensors. Still doing the same thing. After a little more checking I found out that 2, 4 and 6 were firing but 1, 3 and 5 have got nothing. A little more work and I got 5 to start working 1 and 3 are not getting anything. Could you maybe tell me what could cause this and where to go from here? I am stumped.