Powering ATtiny85 for temperature sensing

I have been working on a miniature temperature sensing module which buzzes once the temperature crosses a threshold using ATtiny85, lm35 and a pico buzzer. When I power the whole circuit using an arduino board and take readings of the temperature sensor, they are consistent. But when I try to take the readings after powering the whole thing using 2xcr2016 3V coin cells, the readings are erratic. How do I solve this issue?