AT-Mega 2560 analog pins
So I hooked up a 10 DOF sensor [...] Now all the temperature sensors give 21 C temperature reading. Not sure what this modification does as I didn't change anything for the MCP9700A.
  [...]
  when I remove Vin to 10DOF chip, problem starts up again [...]


From these updates in the question and subsequent comments, this behaviour fits with insufficient decoupling on the 5&nbsp;V power supply to the MCP9700A devices.

The reason that adding the Adafruit 10 DOF module makes the MCP9700AE readings correct, could be because that Adafruit module includes at least one capacitor on the 5&nbsp;V rail, as the input capacitor to its onboard LDO regulator (see red arrow below). So one side-effect of connecting this Adafruit module to the 5&nbsp;V supply from the Arduino (which also powers the MCP9700A devices) is that it adds this extra capacitance to that power supply rail.


original image source

Therefore, based on the results seen, I suspect that noise on the 5 V power supply to the MCP9700A devices could be the cause of the erratic readings. And connecting the Adafruit module adds enough capacitance to that 5 V supply to reduce noise and prevent the erratic readings from those MCP96700A devices.

Further confirmation would be possible, by adding only decoupling capacitor(s) to the 5&nbsp;V power rail instead of connecting that Adafruit module, and seeing the same stable readings from the MCP9700A.