Bosch random orbit sander head won't stay on

I have a Bosch 3283DVS random orbit sander that has received light use for many years. Suddenly today the head (or whatever it's called) got very wobbly. I pulled off the sandpaper and found that the screw holding on the head was backed out. I re-tightened it.

Upon using the sander again, within two seconds the head again started wobbling. This time I removed the head and examined where the head mounts to the, um, whatever you would call it. Eccentric plate? I saw that there were three protrusions on that plate, and three holes in the head. Oops, OK, I need to line those up. I carefully lined them up and re-tightened the screw. But no joy, it immediately came loose once again upon starting back up.

There is a lock washer on the screw. It looks OK, but I guess I could replace it.

I could try thread lock, but there's no evidence of any there now.

Any suggestions? Am I missing something here? And how long does it take thread lock to harden up?I need to get on with this work :-)