Thanks again !
I have 100 nF capacitors near to power pins of PIC and MCP ... I use a Sony player for tests ; I don't have a serial display.
As I said, my previous attempts (using PIC ADC) worked very fine in Proteus, worked very fine "on work table", but not so well on the car : sometimes the buttons wouldn't react properly (maybe because of automotive environment ?).
I used LM2931-CZ5 for power supply, 100 nF ceramic capacitors, even a 100 uH coil before the voltage regulator. The part of code which reads the values of ADC is the same in this code...