I am bumping my own thread because I have news :
I was wrong it wasn't a case of dirty reels and tank, the culprit was the developer! And more precisely the water used for mixing it. It was bloody hard and for a reason it made the developer foam in the tank.
I realized this by accident when I decided to stop buying Kodak's D-76 and make my own. The first batch I mixed from raw chemicals was cloudy and milky and that's how I realized my water was not good.
(the funny thing is my D-76 made from Kodak bags was always limpid... I guess they add a few things to the raw chemicals at Rochester).
The plot thickens now : my "hard water" cloudy milky D-76 always gives me foam and bubbles on the negs... except when it's rested for a few days (say 10). After a short period it's much less troublesome. Go figure!