Your occasional auto-shifts may be due to the shifter. If it didn't get done during the service, you might try to tighten it up a bit with the plastic wingnut. Be carefull, though, because all that plastic Simplex stuff gets fragile with age. Also with those shifters, if you go to replace the cables, the lead head that fits into the shifter is smaller than on Shimanos -- maybe cables specified for Campagnolo shifters will be a better fit. I know that about 10 years when I was messing with those shifters on a Peugot there was a problem getting the lead into the shifter, but can't recall what I did about it.
I know you're not going to replace your shifters, but if in the future you think about another approach, the Suntour mountain bike thumb shifters from the 80s, I think, work pretty well and would fit a 22.2 mm bar.