Can any one suggest a possible cause????

Problems with the PIC 16F84 -10/p.

I have been running the 16C84 -4/P for some time now without any problems,
I
then started to use the 16F84 -4/P as a replacement for the 16C84 -4/P
again no problems, running the same code in the same circuit at the same
clock speed 4Mhz. I then replaced the 16F84 -4/P for the 16F84 -10/P, this
change caused the program to crash as soon as it reads or writes to the on
broad EEPROM area.
If I go back to the 16C84 -4/P or the 16F84 -4/P in the same circuit, clock
speed, program and programmer ( PIC Start Plus) no problems occur.
I have also tried the 16F84 -10/P with code that does not use the EEPROM
area and all works fine, this is using the same PIC 16F84 -10/P as used
with
the code that reads and writes to EEPROM,
So this points to the fact that the microcontroller has not failed and
still
runs code that does not require  it to read or write to EEPROM.

If you could help out with this problem as I have just purchase 25 16F84
-10/P and I'm using the EEPROM area on most of my programs.