I tried using 2 diodes (1N4007) but did not work. I also tried to use signal diode 1N4148 but failed. Finally tried with two fast diodes (schotky) and did not work!
For now the best way of functioning was linking directly (1k resistor between tx(5v) and rx (3,3v) but I know this is wrong and yet sometimes fails!
Anyone have any more ideas?