I had a Peugeot that had what I called "chain droop", where it would pedal fine, but when I would stop, the chain slack would not be taken up and the chain would literally fall off of the front chainring. Very disconcerting. I tried several things, thinking it was the RD, then one day told a guy at my local LBS what was going on and he said, "Oh, a sticky freewheel", like it was very common. Probably is, but it was the first time I'd had it happen. What he recommended was to flush the freewheel out with penetrating oil or WD40 (I used penetrating oil) using the straw attachment. With the wheel on its side so that the freewheel is flat,I shot the oil into the small gap in the freewheel and then let it all run out. I spun the freewheel several times and then put the wheel back on. Perfect, problem fixed. He then recommended some light oil as a lubricant. I ended up using 5W-30 motor oil and it was fine for years. You might give it a try.