Short update (don´t ask me the entire story please). It looks like the drift is gone (I think the reflow helped here) BUT the 100R 2W adjustment process does not work.
Basically before and after the 100R 2W adjustment the meter is reading pretty much the same value out of tolerance.
I allow myself to declare the issue is not an analog one (that´s why I was going in circle without huge steps forward) rather a digital one (bad cell memory on the 2W 100R value??, it won´t overwrite that value anymore???).
Anyway I am close to put that unit on ebay as not working (just because I like to be honest), since 99% is working perfecty and only the 100R 2W is slightly out tolerance I think somebody of you guys can make a good deal.
A friend of mine is an exorcist, but he told me he won´t deal with electronic devices as a 34401A