When basal body temperatures fluctuate a great deal, it is usually due to inconsistencies in temperature taking methods. Make sure you are taking your temperature at the same time every day, when you first wake up, after a solid block of sleep, and with a BBT thermometer. If you are doing all this and are still getting rocky temperatures, you can try taking your temperature vaginally. Some women notice more stable temperatures when taking their temperature vaginally.
Temperatures can also fluctuate a great deal when you are not ovulating, or when you have cycles with delayed ovulation. If you are taking your temperatures accurately and are still having large temperature fluctuations and no clear ovulation pattern over a few cycles, this should be mentioned to your health care provider, especially if it is taking a long time to conceive.