Raspbian cron跑得太晚了

On Raspbian, I have setup a script to run every minute through crontab -e: * * * * * /path/to/php /path/to/script

The script is a temperature logger. It reads the temperature from a sensor and writes the result to a (mysql) database. A couple of times a day it doesn't start the script for about 8 minutes. Then the script is being started all the missing times. For example, the log shows me these times:

  • 12.00.05 (normal)
  • 12.01.06 (normal)
  • 12.02.05 (normal)
  • 12.10.08 (7 minutes late)
  • 12.10.08 (6 minutes late)
  • 12.10.08 (5 minutes late)
  • 12.10.09 (4 minutes late)
  • 12.10.09 (3 minutes late)
  • 12.10.09 (2 minutes late)
  • 12.10.09 (1 minutes late)
  • 12.10.10 (normal again)
  • 12.11.05 (normal)

There aren't any crons that run around the time the failures occur. What can be the problem and how do I find or fix this?