Sir ... you got it !
I use to "play" with a homemade dev board with a lot of stuff things
like LCD, switches, push button, led, In situ prog, etc ...
this developpement board is OK for normal use.
In this case .. I discover that the small power supply has a 0.120 volt
ripple .. with some spikes !
2200 µF on the power itself, the well know trio of decoupling capacitors
near the +5 volts regulator, and also near VCC and VDD on the PIC !
you got it !
I have forgotten the basic rule of electronic .. have a CLEAN power supply!
it was clean for basic opération since 5 years ! not for analogique use !
you are the winner !

I am working on the following project :
Using an On Screen Display IC (STV5730) show the antenna rotor position
used in a radio amateur TVA repeater.
I need to find a very good potentiometer to avoid variations in the reading
thanks a lot 73's from F1CHF