I made the change to the interrupt as suggested by Udo and the clock is now running reliably. Thank you very much!
@Nick, I am thinking I will continue to use the 32.768khz crystal which seems to be fairly accurate. I require the Arduino to wake up only a few times per day so the sleep intervals are fairly long so I think I will stick with my existing method of incrementing a variable. I have a GPS connected so I am thinking I can use the time from the NMEA data to update the seconds variable and keep everything running accurately. That is my plan anyway, we will see how it works out....