Finally today I and my hifi guru we have fixed the issues concerning
PL 4000 switches .
After disassembling the front panel we cleaned
all the switches with CRC 2-26 , but the problem always
occurred random when you moved some switches .
Anyway after 3 hours my hi fi guru realized
that there were many bad contacts between mother board
and front panel board , " amplified " by the operiations of the
switches .
Solution : clean and resolder the male connector on the front panel board .
Now everything looks O.K.
Probably these bad contacts are due the flexion of the front panel board .