Hi, reading the posts is it right in stating that the readings you get are inaccurate not unstable when you install another sensor.
Where are you getting your power supply for the sensors, I hope not from the arduino 5V. If this is the case then have you tried a separate power supply for the sensors?
You were asked about what the supply was measured at your sensors and I gather the 4.70 to 4.75V stated was the result.
It should be 5 volts if its coming from a regulated supply, again if its the 5V on the arduino board that you are using then I'd say you are overloading it regulator.
Tom.