Well, took a few hours this afternoon to change the automatic trans filter + fluid. Since the CSS was working fine as I was doing errands to get everything up to operating temperature, I decided against taking off the switches and see whether the fluid and filter change would make a difference. After doing the whole procedure (which took me longer than I had expected, mostly because of all of these bolts that hold the transmission oil pan in place, plus the fact that I dropped my wrench into the drain pan full of hot transmission fluid a couple of times!) I test-drove the car and the erratic behaviour is still with me. Most of the time it works, and I can hear the switch clicking actually, that's how quiet the car idles now with the 123 ignition in place (see other thread on that) when it does engage as I put the car in 'D', but sometimes it doesn't. So next step is, when I have time again, removing the switches.
My wife, 9-year old son and I are off to our country house in France tomorrow (via Disneyland Paris), where I have two spare auto transmissions. I will take two of these switches off of those, and use them as replacement(s) for my current (again, pun intended) switch(es). Or maybe the forward switch just needs cleaning; I noticed fine metal shavings in the oil pan (brrrr....) when I changed the filter etc so maybe it's 'just some debris' (bbbrrrrrrrr.........) in there!
To be continued.