Thanks for those firsts advices.
At the first time, we have found some bad solders, but probably
not all the ones.
We have remplaced the electrochimic capacitors in the main power
supply, they were very "tired".
When the problem arrives, the reception is cut on all bands but
sometimes, the frequency counter continue to display good numbers
only on some bands.
We're going to continue to climb this mountain, meter by meter.
Thanks.

Guillaume.