Temperature sensor TMP36 Arduino problem
I had a similar problem to this with the LM35, which was caused by the LM35 oscillating due to a long cable length between the sensor and Arduino. I fixed this by adding a 100Ohm Resistor to the signal line.

Changing to the TMP36 also fixed this problem for me because it is more robust when driving capacitive loads. But even so, the datasheet recommends a resistor for large capacitive loads.

Also don't forget the bypass cap across the supply.