Engine Skip

You need to isolate the skip. Pull plug wires one at a time until you get to the cylinder that makes no difference. Look at the plug real good. I've seen more than once where the center electrode was broken. At idle and low RPM, it would fall against the ground electrode and ground the plug. At higher RPM, the compression would blow the broken center electrode up and it would fire. Also too, could be a weak or broken valve spring.