Well, I looked at my truck and I don't see any way that the clutch and throttle could be related in such a way as stepping on the clutch could affect the throttle linkage.
I think it must be a coincidence. Maybe when you're backing off on the gas pedal to work the clutch, the rod is binding in the firewall or something similar.
As you said, lubricate any connections and look at where the rod goes through the firewall. What's the story on your throttle return springs? Are they connected properly?
