Hate to bring some real engineering practice into it...
Variation is about 5% between 3V and 5V Vcc (eg. when powered with 3xAA batteries).Variation is about 5% for every 20 degrees difference in temperature (a difference easily possible from summer to winter).
5% variation is enough to make it fail.
I did an experiment with SoftwareSerial
How can i learn what is the right value for my attiny that will be compared from the tuner?
Is this a fuse that i try to tune?