AVR-Chat

Additional suggestions (I've been playing with a 'mega8 but have not
employed the ADC yet):

You are always better off with a larger Vref, since 1/1024th of Vref
is your LSB. Your analog system noise should be an order of magnetude
lower than this.

There should be a low pass filter (inductor and capacitor) on the AVcc
pin to reduce Vcc noise coupling from the digital circuitry.

The ADC is designed for analog sources of 10K or lower (is better)
output impedance. Your 100K pots might be allowing a little more
source-noise into the system. You might try an experiment by placing a
small capacitor from the analog input pin to ground (should be fine
since you are using a pot). Short traces with good ground planes and
isolation from other noisy signals are a MUST at 10 bits and greater.

Aref should not be connected to anything except an external cap in
normal use. If you are referencing your pots to this you must buffer
this with a high impedance buffer amp.

You didn't say if you were using the "ADC Noise Canceller" (sleeping
the rest of the chip) during conversions. From reading the doc, it
sounds like its really necessary. The down-side is that you can only
run in single-conversion mode.

Cheers,
Paul H.