2Tricky:
Lefty just in case I've negated a negative (!?), the first implementation displayed similar wobble without the function 'analogReference()' being called.It's interesting what you say about alternative chips and I wonder whether you have any models in mind. I admit that I'm trying to do this on half a shoe string, but a few quid of prudent expenditure is surely horses for courses.
Thanks for your view...
The only one I've had first hand experience with on a arduino is the TI ADS1115 16 bit 4 channel (or 2 differential channels). http://www.ebay.com/itm/4-Input-16-Bit-I2C-Analog-Digital-Breakout-Micro-Controllers-Arduino-/330761041957?pt=LH_DefaultDomain_0&hash=item4d02e4f825
I have a check out sketch code available if you actual try one of these out.
Lefty