Welcome. :thumbsup3:
I had a similar situation when I changed out the wires and plugs on my '66 (I know, it's not a C5) and all it was was that I didn't push hard enough on the wire cap on one of the plugs. That was an easy fix for me but since I did nothing else to warrant rough idling, and shitty performance, I figured the connection between wires and plugs had to be the culprit. Hope it helps.
I had a similar situation when I changed out the wires and plugs on my '66 (I know, it's not a C5) and all it was was that I didn't push hard enough on the wire cap on one of the plugs. That was an easy fix for me but since I did nothing else to warrant rough idling, and shitty performance, I figured the connection between wires and plugs had to be the culprit. Hope it helps.