Thanks guys.
I will try a few more Serial.print commands hoping that they are not what causes the problem. (Come to think of it, I will try it without Serial.prints to check this theory).
@ Coding Badly
The unit will be solar powered on a hilltop so may experience 0 - 50 degrees Centigrade inside its enclosure depending on the time of day and angle of the sun, so I would have trouble tuning it to a specific temperature. I might resort to a crystal after all, even though they use a bit more power.
Thanks for the heads up on the serial speed. I have been on the wrong track there.
What is the purpose of the clock divisor?
What is the difference between 8 Mhz divided by 8 and using 1 Mhz without divisor?