I have a 98 Blazer 4X4 and I'm getting a misfire code on cylinder 4. When I put a spark tester on it I get spark @ idle. As soon as you give it some gas it loses spark as RPM's increase then starts getting spark again as soon as the RPM's start to come down. It has brand new plugs, wires, cap, and rotor. I have tried replacing the ECM and ICM/Coil with ones that are know to be good. Any ideas would be a big help.

Thanks
Brett