This is a recent development: my Creator Pro has started losing track of its Z starting position. I will launch a print job, the platform comes up to starting position, and then it makes this awkward tick-tick-tick noise, hitting its vertical end stop without knowing when to stop. After that, we can be fairly confident that my Z positioning is badly off, and the model will print too high or too low. Usually too low, practically grinding the plastic into the plate.

Until recently I could get around this problem by giving the "Home axes" command from the menu before printing. But now even that command is having the same problem.

Clearly some piece of hardware is failing. What piece could it be?