Thanks to all of you. All were good suggestions and I have implemented them all. Replacing the WRITEs in the ISR with a FLAG that is polled in the main loop to determine whether to WRITE the time stamp to EEPROM, fixed the problem.

Sayez's idea of using a step variable and LCD to troubelshoot where in code a problem exists is a great idea I had not heard before. Will definitely make use of it in future. Thanks!

This forum is one of the greatest resources that exists in the microcontroller design world. Thanks to all of you for your great participation and contributions!
John Ellis