Hi,
Just a small note to say I found the issue power supply versus USB. In my case it was the wall wart adapter, it was a switching mode power supply. It had some ripple on it, just enough to cause the adc readings to fluctuate.
With which resistance I may get the best results?
So basically we're back at the beginning of this thread and we can't answer this specific question for you, since you need to measure, test and look for a value which will give you the range of sensitivity you want.
Do you understand that the sensor and the load resistor act as a voltage divider? Once you grasp this, it's easy. The formula to calculate the voltages/current/resistances in a voltage divider is very basic. See Voltage divider - Wikipedia
Or if you don't like to calculate, just buy a bunch of trimpots and find it by trial and error. It's a proven method 
By the way, if you like to measure the value of the sensor with no load with your current multimeter, just put the 1Mega Ohm resistor in parallel to the sensor and measure the parallel resistance, it will be less than 1 MOhm so your multimeter can handle it. Afterwards you can calculate the actual value of the sensor resistance
Regards,
Carlos