I had a similar problem with one I had from a while back. The clock was always working fine, but the optos for the magnets, ramps, and upper pf would go out randomly. I could just sit there in test mode without touching a thing and the optos would start acting up. It turned out to be cracked solder joints on the opto board under the pf. The board was also filthy, so I took it out, cleaned it up, re-flowed solder and it worked fine ever since.

Does that board look completely clean or does it have any burnt spots? Also, how do the caps look, are any bulging or look abnormal? If you know anyone close to you who has a TZ, I would try switching that opto board first and see if the problem persists. That way you can at least start to narrow down where the issue is stemming from.

I see you also have a RS. I know RS and TZ are in that same superpin family (or whatever it's called), but does RS have one of those under pf opto boards?