groundFungus, thank you for your time and patience!
Using this plug/unplug method, I now get steady uploads. I believe my wiring correlates with yours, which makes me wonder about that 10K resistor - it is very old and the colours are so faded that I am not confident that it is indeed 10K (don't know how precise this needs to be?). I live away from civilization, so it will take a bit of time to get replacements.
Anyway, I am on my feet again, and hopefully this brings me to the last stretch of a project that took more than 3 years, a security network consisting of sentry guns with PIR and radar, and some other gimmicks 
Thank you very much...