Another thing - do you have 0.1uF cap on pins 11 & 20 to ground on every device? That will help to keep switching noise out of the picture also.
Caps are also recommended from Aref pin to gnd for stability also.
"The internal reference can source up to 1 mA, therefore. it can be used as an external system reference.
A decoupling capacitor must be connected to the REF pin if internal reference is selected to ensure output stability.
A 1 mF to 10 mF capacitor in parallel to a 100 pF capacitor should be sufficient, see Figure 13."