A friend of mine suggested me that the issue could be software related: variable stucks, non predictable behaviour... and perfect voltages. I then decided to place the old v2.13 firware the synth came with. Well, it's working now and I have no issues now. I will toy with the unit for so e day and see if i found the solution.
Have anyone here ever had strange issues with tauntek firwwares I burned two of those v2.15 firmwares on different 27256 eproms and both are defective someway, even if verified...