Interesting! Sounds like to just need to keep an eye on the situation.
What state is the camshaft in, I wonder? Smooth, clean (dry)? Do you lubricate them at all? My other thought is about the door (pressure bar springs) ... but you're saying that it's all OK for a few hours initially.
But I would still suspect the set(s) myself.
But when you think about it, it's a wonder that pumps without a drop sensor are accurate at all, especially over time!
As I have mentioned before, in the "perfectly designed pump", all the sensing would (should) be done as close to the infusion site as possible. That is, what is actually being delivered (and at what pressure), and then applying feedback. Rather than by counting camshaft revolutions with an optical encoder (or whatever).
