alkopop79:
Since my last post I've been experimenting with different pull-down resistor and pot values. Unfortunately, whatever resistor I use the connections effect each other.
If you are using an external multiplexer you MUST add a time delay between selecting an input and analogRead(). The Arduino does that automatically when you are using the INTERNAL multiplexer but doesn't if you use an EXTERNAL multiplexer.
I recommend a 100k pull-down resistor on each input and a low-impedance voltage source such as a 1k pot. Set up your multiplexer address, delay one millisecond, and do the analogRead(). If that works you can reduce the delay to hundreds of microseconds.