Help with Hardinge program

I wrote a small program to loop on a Hardinge 42 with Fanuc O control. It works fine for say 45 to 60 minutes then stops in cycle without any alarms. Reset program and starts again fine till around hour later with same problem. Its like there is a parameter that limits loops to a set number. Here is the program ( written to exercise axis.)

M03 S1000;
G00 X253.0 Z290.0;
G04 S5.0;
M04 S1000;
X50.0 Z50.0;
G04 S5.0;
M99;

Any ideas???