"Clamp To" drifts over time

I’m just starting to experiment with animation in Blender 2.57, and one of the main things I’m doing is setting up a path for the camera to follow. The Clamp To constraint seems to be the way to go, but it doesn’t seem to actually clamp the camera to the path properly.

Over time, the camera drifts from the path and then occasionally seems to realise it’s drifted, and “jump” back to the path. The influence is set to 1.0, so that’s not the problem (and even with the influence set lower, the camera still jumps at the same points!)… anyone know what’s happening here?

Im having the same problem, except that in this case the camera abruptly jumps off of the curve and then slowly goes back to it.