Question On Setting Timer Interrupt

Hi all,
I'm writing a program to use Global Interrupt to run a clock countdown. The other day I flash on PIC16F877 and it works but then when I use PIC16F877A, things seems to go wrong. the clock reset when the second digits reach 8.

When I set the time to [00:07], it still can perform countdown but not more than [00:08] and above, which makes me wonder if I use the interrupt correctly?

Does anybody know about this?

I paste a portion of the related command below:

----------------

__CONFIG(0x3ff1);

#define PERIOD 1000000
#define XTAL 3580000
#define ICLK 16


void interrupt timer0_isr(void) @ 0x10
{
s14++;
T0IF=0;

}

time_edit(x)
{
if(edit==1)
{
if(edit_mov==0)
minute10=x;
//1 RC4=1;
else if(edit_mov==1)
minute=x;
// RC5=1;
else if(edit_mov==2 && x<6)
second10=x;
// RC6=1;
else if(edit_mov==3)
second=x;
// RC7=1;

grid[6]=sevenseg[minute10];
grid[7]=sevenseg[minute];
grid[8]=sevenseg[second10];
grid[9]=sevenseg[second];

dec_delay();
}

return(x);
}

main()
{
PSA=0;
PS0=PS1=PS2=1;
T0CS=0; //select internal clock
T0IE=0; //enable timer interrupt
GIE=0; //enable global interrupts


ADCON1=6; //PORTA as digital
TRISA=0; //all PORTs as output
TRISD=0b00001111;
TRISB=0;
TRISC=0;
TRISE=0;

PORTE=PORTD=PORTC=PORTA=0; //clearing PORTs

//TRISC6=TRISC7=1;

ADDEN=0; //disable address detection
BRGH=1; //high baud rate
SPBRG=23; //set the baud rate
SYNC=0; //asynchronous
SPEN=1; //enable serial port pins
CREN=1; //enable reception
SREN=0; //no effect
TXIE=0; //disable tx interrupts
RCIE=0; //disable rx interrupts
TX9=0; //8 or 9-bit transmission
RX9=0; //8 or 9-bit reception
TXEN=1; //enable the transmitter

if((second!=0)||(second10!=0)||(minute!=0)||(minute10!=0))
{
if(s14>13)
{
s14=0;
second--;

if(second>250)
{
second=9;
second10--;
}
if(second10>250)
{
second10=5;
minute--;
}
if(minute>250)
{
minute=9;
minute10--;
}
if(second>250)
{
minute10=9;
}
grid[6]=sevenseg[minute10];
grid[7]=sevenseg[minute];
grid[8]=sevenseg[second10];
grid[9]=sevenseg[second];
}
}
}

Thanks for your reply.