Working on an 02 Yukon, 5.3l. Has steady miss, #7 and 8 dropping on cylinder balance. P0300 is only code that sets. Both cyls have spark, fuel, injectors are firing, good compression. #7 smooths out at higher rpm but 8 is dead. Checked rockers and pushrods, swapped coils and injectors to different cys. and miss stays with 7 and 8. We are a Ford dealer so dont see to many of these. Any help is certainly appreciated.