OK, I managed to pull the clutch pedal cruise control de-activation switch. I can see why it wasn't working well - the plunger is very "lazy" - unlike the brake pedal's de-activation switch, which snaps smartly to the fully extended position the moment the brake pedal is pressed far enough to allow it to do so, the clutch's switch's plunger barely extends a few mm. When I had it out, I could make it pop to a more fully-extended position, by depressing the plunger, and then pulling my finger away very abruptly. But when I pull my finger away slowly, it barely extends. So, I would guess the spring inside the switch has given up its ghost.
But also, and this is strange, the resistance across the two connectors reads zero, no matter what position the plunger is in. I would think that given its intermittent behavior, I could get it to go from fully open to fully closed by manipulating the plunger, but no matter what I do, it reads a dead short. So, it looks like I'll be ordering a new one, unless someone has any other ideas. Thanks.
Oh, and I sprayed the switch with contact cleaner, but unfortunately, this didn't change the switch's behavior, either mechanically or electrically.