It's possible that the interrupter on the clock hand isn't consistently breaking the opto. Try extending it a bit with some black tape or something similar.

I'm replying to this old post because I was recently able to fix a problem with my TZ Clock. The solution that worked for me was posted on Pinside embedded in a long thread with a lot of other suggestions; however, I can not find the original thread where I read it: Replace the motor; however, I believe the motor does not need to be replaced, just cleaned.

The symptom is that everything about the clock seems to work during test mode, but in gameplay, the running of the clock is not consistent. In test mode, it may seem to run fine, but a sign of a dirty motor is that the clock will stop running during the test mode. I initially thought that when it stopped running in test mode, it was due to a protective feature so the motor doesn't burn up if left running in test mode, but I don't think this is the case. I think it means the motor is not performing as it should and so it shuts off. At any rate, most times the clock would not work at all and "broken clock" was reported.

Cleaning that worked for me: I take pinball motors apart all the time to clean them up. There are holes in the top to insert pins so the electric brushes can be held in place when putting the top back on. For this motor, the crimping that holds the cap on was a little tight so I decided that my normal cleaning doesn't really need disassembly, so I squirted a lot of isopropyl alcohol into the holes to make sure it had a thorough rinsing off of built-up brush-wear residue.

Now my clock runs faithfully in all timed modes, not just clock chaos or clock millions.