Alain, I didn't post code, schematic, or board because all three work just fine with a 16F88. Now, except for controlling the output pins (which Darrel solved in http://www.picbasic.co.uk/forum/showthread.php?t=12739), the only issue is this "bias" (or whatever) on the AN channels.

Melanie, I've tried to be sure I've turned off the comparators, the CCP modules, and the capacitive sensing module. I don't know if I've missed something, but here's what I did:

ANSELA= %00011111
ANSELB= %11000000
CCP1CON=%00000000
CCP2CON=%00000000
CCP3CON=%00000000
CCP4CON=%00000000
OSCCON= %11110000
TRISA = %01111111
TRISB = %11000001

ADCON1.7 =1
CM1CON0.7=0
CM2CON0.7=0
CPSCON0 =0

DEFINE ADC_BITS 10
DEFINE ADC_CLOCK 6
DEFINE ADC_SAMPLEUS 12

I'll take a look at the Vref matter; maybe that's it. The pots seem okay, with good linear travel and value proportionate to rotation. Remember, I'm seeing the effect with the pins strapped to ground, too.