Hi everyone! First of all PROBLEM SOLVED!!!
Right after i post my second post here i found the problem!
VOLKERWASSERMAN you were very close...and by the way thanks for the replay. It was not for vibration problems but the problem was indeed the DT50 Volume pot at chanel A.
I say that the vibration was not a problem because that happened with no sound, no vibration at all!! Winth nothing playing, and no signal input the problem was the same.
What i've done after lots of tests, flashings, cables, and so one, i decided to do a spray cleaning to the DT50 volume pot!!! and VOILA!!!! PROBLEM SOLVED!!!
Some dust can really make us nuts!!!!!
ruidemoura's post in Hd 500 Volume Goes Totally Down By Itself was marked as the answer